Update .NET version & drop setup steps to fix CI

Updated .NET9.x minor version and dropped setup-.NET steps in
GitHubActions in order to fix CI build of the `docs` job.

Somehow, on the same day as the .NETv10 release, our CI started
failing without changing anything (thanks to our
`on: schedule: cron:` trigger we detected this, as it runs
daily), but only in the docs job. After some discussion about
Fornax in PRs 770 and 779 (given that we updated this dep in
PR#767 recently), it was discovered that the default .NET
version of current GitHubActions agents works out of of the
box without needing to run any setup-dotnet action.

So then, rather than just removing the step in the docs job,
let's remove it from all of them to be consistent; and we
might revisit this decision in the future if/when the
default versions change again (which might make CI jobs
fail again).

Co-authored-by: Richard Webb <richard.webb@helpsystems.com>

Author: knocte

.github/workflows/build+test+deploy.yml

@@ -27,10 +27,6 @@ jobs:
 
     steps:
     - uses: actions/checkout@v4
-    - name: Setup .NET
-      uses: actions/setup-dotnet@v4
-      with:
-        global-json-file: global.json
     - name: Restore tools
       run: dotnet tool restore
     - name: Restore dependencies
@@ -49,10 +45,6 @@ jobs:
 
     steps:
     - uses: actions/checkout@v4
-    - name: Setup .NET
-      uses: actions/setup-dotnet@v4
-      with:
-        global-json-file: global.json
     - name: Restore tools
       run: dotnet tool restore
     - name: Build
@@ -104,11 +96,6 @@ jobs:
 
     steps:
     - uses: actions/checkout@v4
-    - name: Setup .NET
-      uses: actions/setup-dotnet@v4
-      with:
-        global-json-file: global.json
-
     - name: Restore tools
       run: dotnet tool restore
     - name: Restore dependencies

global.json

@@ -1,6 +1,6 @@
 {
   "sdk": {
-    "version": "9.0.201",
+    "version": "9.0.306",
     "rollForward": "disable"
   }
 }