chore: upgrade firebase backend from java8 to java21#10162

Alesvoy wants to merge 2 commits intoGoogleCloudPlatform:mainGoogleCloudPlatform/java-docs-samples:mainfrom

Alesvoy:abutron-chore-migrate-java8-to-java21-firebase-backendAlesvoy/java-docs-samples:abutron-chore-migrate-java8-to-java21-firebase-backendCopy head branch name to clipboard