feat!: remove Python 2.7 from tested versions (#13675)

* feat!: remove Python 2.7 from ALL_VERSIONS

* remove app engine forked noxfile-template

This file should be no longer required since the only difference was 2.7

* remove 2.7 configurations

Author: glasnt

.kokoro/python2.7/common.cfg

@@ -88,7 +88,7 @@ def get_pytest_env_vars() -> dict[str, str]:
 
 
 # All versions used to tested samples.
-ALL_VERSIONS = ["2.7", "3.8", "3.9", "3.10", "3.11", "3.12", "3.13", "3.14"]
+ALL_VERSIONS = ["3.8", "3.9", "3.10", "3.11", "3.12", "3.13", "3.14"]
 
 # Any default versions that should be ignored.
 IGNORED_VERSIONS = TEST_CONFIG["ignored_versions"]