Drop compatibility with 6.0.0 (#4411)

**What type of PR is this?**

Cleanup

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

Simplify C++ toolchain handling by requiring at least Bazel 6.1.0. This
will be used in future PRs to implement proper, exec-platform aware
resolution for the combination of Go and (optional) C++ toolchain.

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

**Other notes for review**
3 files changed