chore: Update source target to 1.8 (#724)

Updated maven source config to 1.8
Updated the CONTRIBUTING.md to reflect Java 8+ support.

Author: lahirumaramba

CONTRIBUTING.md

@@ -85,9 +85,7 @@ information on using pull requests.
 
 ### Initial Setup
 
-Install Java 7 or higher. You can also use Java 8, but please note that the Firebase Admin SDK must
-maintain full Java 7 compatibility. Therefore make sure that you do not use any Java 8 features
-(e.g. lambdas) when writing code for the Admin Java SDK.
+Install Java 8 or higher.
 
 We use [Apache Maven](http://maven.apache.org/) for building, testing and releasing the Admin Java
 SDK code. Follow the [installation guide](http://maven.apache.org/install.html), and install Maven

pom.xml

@@ -272,8 +272,8 @@
                 <artifactId>maven-compiler-plugin</artifactId>
                 <version>3.10.1</version>
                 <configuration>
-                    <source>1.7</source>
-                    <target>1.7</target>
+                    <source>1.8</source>
+                    <target>1.8</target>
                 </configuration>
             </plugin>