Update apigee keystore resource names to be consistent

[modular-magician]

GoogleCloudPlatform/magic-modules#7518 was released in https://github.com/hashicorp/terraform-provider-google-beta/releases/tag/v4.60.0 adding google_apigee_keystores_aliases_key_cert_file. That resource is a fine-grained resource of https://cloud.google.com/apigee/docs/reference/apis/apigee/rest/v1/organizations.environments.keystores.aliases, a custom endpoint that supports multiple different kind of aliases.

That resource probably should have been called google_apigee_keystore_alias_key_cert_file, or google_apigee_env_keystore_alias_key_cert_file based on google_apigee_env_keystore. See discussion in GoogleCloudPlatform/magic-modules#7472 (work on another of those FGRs that was started before that change, but took longer to finish).

4.61.0 is set to include:
* google_apigee_env_keystore_alias_pkcs12 from GoogleCloudPlatform/magic-modules#7519
* google_apigee_env_keystore_alias_self_signed_cert from GoogleCloudPlatform/magic-modules#7488

However, these are subresources of the same resource. This PR corrects them to be self-consistent with the released resource (google_apigee_keystores_aliases_key_cert_file). Therefore the new resources will be:

• google_apigee_keystores_aliases_pkcs12
• google_apigee_keystores_aliases_self_signed_cert

I'm not sure how to catch these situations faster- there were multiple PRs, multiple authors, multiple reviewers, and PRs took different lengths of time to go through review.

If this PR is for Terraform, I acknowledge that I have:

• Searched through the issue tracker for an open issue that this either resolves or contributes to, commented on it to claim it, and written "fixes {url}" or "part of {url}" in this PR description. If there were no relevant open issues, I opened one and commented that I would like to work on it (not necessary for very small changes).
• Ensured that all new fields I added that can be set by a user appear in at least one example (for generated resources) or third_party test (for handwritten resources or update tests).
• Generated Terraform providers, and ran make test and make lint in the generated providers to ensure it passes unit and linter tests.
• Ran relevant acceptance tests using my own Google Cloud project and credentials (If the acceptance tests do not yet pass or you are unable to run them, please let your reviewer know).
• Read the Release Notes Guide before writing my release note below.

Release Note Template for Downstream PRs (will be copied)

No changelog entry- this will be cherry-picked to `4.61.0` and affects unreleased resources. The changelog will need to be amended, though.

Derived from GoogleCloudPlatform/magic-modules#7665