Remove CentOS7 fallback logic and clean up configurations (#2630)

Follow up on the migration done in #2272: I Verified via the
gs://bazel-centos-deprecation/ GCS bucket that **no** CentOS7 platform
mapping has been triggered in over a year.

* Removed the deprecated CentOS 7 platform redirection in bazelci.py and
bcr_presubmit.py.
* Deleted the unused log_deprecated_platform_usage function.
* Updated the postsubmit-skymeld task to Rocky Linux 8.
* Stop publishing Linux release binaries to legacy centos7 and
ubuntu1404 buckets.
* Deleted bazel-release-arm64.yml file, which was forgotten after
deleting bazel-release-arm64 pipeline from Terraform
[here](https://github.com/bazelbuild/continuous-integration/commit/e23c007c049ada891fe9caf2b690bd68802c949b#diff-eadc17d6dde68dd2fb3a36c531c6172a9a94d4d7004bdfe9b1f418c0fa639174L189).