Spring codegen: update and simplify generator logic after recent changes in gapic client generation

For
current logic [here](https://github.com/GoogleCloudPlatform/spring-cloud-gcp/blob/70edcc8d0eaafdf3702eb2ff1fa34d9a31590f73/spring-cloud-generator/scripts/generate-library-list.sh#L79-L83) grabs googleapis_committish for each module to generate. The generate scripts:
- looks for the google-cloud-java release tag
- looks at git history under path of each module/version to generate, find latest relevant change before the release tag
- Then for each library, use its commit to checkout versioned protos folder to setup bazel rules ([here](https://github.com/GoogleCloudPlatform/spring-cloud-gcp/blob/3e39459d45e60e65ae684a22e8bc1ff91b24d799/spring-cloud-generator/scripts/generate-steps.sh#L97))

After hermetic build implementation, the monorepo generates with the same googleapis commit ([config](https://github.com/googleapis/google-cloud-java/blob/36307f243d20fb72f8390711c555c9f788881d72/generation_config.yaml#L3)). This generation logic should be simplified.

cc.@burkedavison