chore: add java 21 profile

[mpeddada1]

As discovered in #1975, running mvn clean install in jdk 21 is results in the following error:

INFO] Compiling 157 source files with javac [debug target 1.7] to target/classes
[INFO] -------------------------------------------------------------
[WARNING] COMPILATION WARNING : 
[INFO] -------------------------------------------------------------
[WARNING] bootstrap class path not set in conjunction with -source 7
[INFO] 1 warning
[INFO] -------------------------------------------------------------
[INFO] -------------------------------------------------------------
[ERROR] COMPILATION ERROR : 
[INFO] -------------------------------------------------------------
[ERROR] Source option 7 is no longer supported. Use 8 or later.
[ERROR] Target option 7 is no longer supported. Use 8 or later.
[INFO] 2 errors 
[INFO] -------------------------------------------------------------
[INFO] ------------------------------------------------------------------------
[INFO] Reactor Summary for Parent for the Google HTTP Client Library for Java 1.44.3-SNAPSHOT:

This is because the parent pom explicitly sets the source/target values in the maven-compiler-plugin and maven-javadoc-plugin to 1.7.

This PR introduces a profile that is activated when jdk 21 is used and overrides the source/target values to 1.8 in the maven-compiler-plugin and maven-javadoc-plugin.