[Gradle 9] Removed minSdkVersion and only use minSdk (flutter#173892)

Support for `minSdkVersion` is completely dropped in Gradle 9, and we
should only use `minSdk`. We also included an AGP version checker to
ensure builds did not crash when getting the minimum sdk version. Since
our minimum AGP version is now 8.3, we can remove agp version check.

## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [x] I signed the [CLA].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [ ] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel
on [Discord].

**Note**: The Flutter team is currently trialing the use of [Gemini Code
Assist for
GitHub](https://developers.google.com/gemini-code-assist/docs/review-github-code).
Comments from the `gemini-code-assist` bot should not be taken as
authoritative feedback from the Flutter team. If you find its comments
useful you can update your code accordingly, but if you are unsure or
disagree with the feedback, please feel free to wait for a Flutter team
member's review for guidance on which automated comments should be
addressed.

<!-- Links -->
[Contributor Guide]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#overview
[Tree Hygiene]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md
[test-exempt]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#tests
[Flutter Style Guide]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md
[Features we expect every widget to implement]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Style-guide-for-Flutter-repo.md#features-we-expect-every-widget-to-implement
[CLA]: https://cla.developers.google.com/
[flutter/tests]: https://github.com/flutter/tests
[breaking change policy]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Tree-hygiene.md#handling-breaking-changes
[Discord]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Chat.md
[Data Driven Fixes]:
https://github.com/flutter/flutter/blob/main/docs/contributing/Data-driven-Fixes.md

Author: jesswrd

packages/flutter_tools/gradle/src/main/kotlin/DependencyVersionChecker.kt

@@ -155,7 +155,7 @@ object DependencyVersionChecker {
             val minSdkCheckTask =
                 project.tasks.register(taskName) {
                     doLast {
-                        val minSdkVersion = getMinSdkVersion(project, it)
+                        val minSdkVersion = getMinSdkVersion(it)
                         try {
                             checkMinSdkVersion(minSdkVersion, project.rootDir.path, project.logger)
                         } catch (e: DependencyValidationException) {
@@ -180,20 +180,7 @@ object DependencyVersionChecker {
 
     private fun generateMinSdkCheckTaskName(it: Variant) = "${FlutterPluginUtils.capitalize(it.name)}$MIN_SDK_CHECK_TASK_POSTFIX"
 
-    private fun getMinSdkVersion(
-        project: Project,
-        it: Variant
-    ): MinSdkVersion {
-        val agpVersion: AndroidPluginVersion? = VersionFetcher.getAGPVersion(project)
-        // TODO(reidbaker): Remove version check as 8.3 is the minimum supported version.
-        // Keeping the check around so that users that bypass will get the error message and not
-        // a compile time error. See https://github.com/flutter/flutter/pull/171399
-        return if (agpVersion != null && agpVersion.major >= 8 && agpVersion.minor >= 1) {
-            MinSdkVersion(it.name, it.minSdk.apiLevel)
-        } else {
-            MinSdkVersion(it.name, it.minSdkVersion.apiLevel)
-        }
-    }
+    private fun getMinSdkVersion(it: Variant): MinSdkVersion = MinSdkVersion(it.name, it.minSdk.apiLevel)
 
     @VisibleForTesting internal fun getErrorMessage(
         dependencyName: String,