prepare rules_go release 0.51.0-rc2 (#4183)

**What type of PR is this?**

Bug fix

**What does this PR do? Why is it needed?**

Deprecates the `RULES_GO_VERSION` constant in favor of `bazel_dep` and
keeps it at `0.50.0` for legacy consumers that may not be able to parse
RC suffixes.

**Which issues(s) does this PR fix?**

**Other notes for review**
2 files changed