Migrated CIs from split repos should be conditionally required #12702

For example, the CI of sdk-platform-java is currently not required. This could cause mistakes of merging in changes with failing tests.

We should make all migrated CIs from split repos conditionally required and always pass if there are no changes to the specific folder.

• showcase (11)
• showcase (17)
• showcase (21)
• showcase (25)
• library-generation-lint-python
• library-generation-lint-shell
• library-generation-unit-tests
• dependency-compatibility-test
• downstream-compatibility-test (java-bigtable)
• downstream-compatibility-test (java-firestore)
• downstream-compatibility-test (java-pubsub)
• downstream-compatibility-test (java-pubsublite)
• downstream-compatibility-spring-generator
• java8-compatibility-check
• Build <- Sonar Cloud
• unmanaged_dependency_check
• build (11)
• build (17)
• build (8) except for gapic-generator-java
• build (8) for gapic-generator-java
• build (8) for showcase
• build (21) except self-service clients
• build (25) except self-service clients
• dependency_analyzer_unit_test
• gapic-generator-java-bom
• showcase-clirr
• unmanaged_dependency_check

Kokoro:

• Kokoro - Test: Auth GraalVM Native Image
• Kokoro - Test: Auth Integration
• Kokoro - Test: BigQuery Integration
• Kokoro - Test: BigQueryStorage Integration
• Kokoro - Test: Datastore Integration
• Kokoro - Test: Integration
• Kokoro - Test: Logging Integration
• Kokoro - Test: Logging Logback Integration
• Kokoro - Test: Spanner Integration
• Kokoro - Test: Spanner JDBC Integration