chore(proc): Update repo's settings (GoogleCloudPlatform#8957)

Configure Java 11 and Java 17 runtime checks as required.
Explain about what checks are run vs. any submitted change.
Provide recommendation about Java runtime version that should be used to build code samples.

Author: minherz

.github/sync-repo-settings.yaml

@@ -6,6 +6,7 @@ branchProtectionRules:
   isAdminEnforced: true
   requiredStatusCheckContexts:
     - 'Kokoro CI - Java 11'
+    - 'Kokoro CI - Java 17'
     - 'Kokoro CI - Lint'
     - 'cla/google'
   requiredApprovingReviewCount: 1

README.md

@@ -48,10 +48,12 @@ To browse ready to use code samples check [Google Cloud Samples](https://cloud.g
 
 ## Supported Java runtimes
 
-The minimal supported Java runtime is Java 11.
-Testing of the code samples in Java 8 runtime is discontinued.
-Code samples that are build with older Java runtimes (e.g. Java 8) are requested to pass tests with Java 11.
-Code samples designated to build and run with Java 8 will be removed at end of support for Java 8 in GCP.
+Every submitted change has to pass all checks that run on the testing environments with Java 11 and Java 17 runtimes before merging the change to the main branch.
+We run periodic checks on the environments with Java 8 and Java 21 runtimes but we don't enforce passing these tests at the moment.
+Because Java 8 is a [supported Java runtime][supported_runtimes] in Google Cloud, please configure to build your code sample with Java 8.
+In exceptional cases, configure to build your code sample using Java 11.
+
+[supported_runtimes]: https://cloud.google.com/java/docs/supported-java-versions
 
 ## Source Code Headers