7.0.0 README updates and minor code changes

Contains many editorial improvements to and some extra information in
the README, along with a few small improvements to the code. Broken out
from bazelbuild#1710, and part of bazelbuild#1482 and bazelbuild#1652.

Specifically:

- Adds more info on translating `@rules_scala` to
  `@io_bazel_rules_scala` for dependencies via repo mapping.

- Merges information about the previously planned `rules_scala` 8.0.0
  release into the information for 7.0.0, since it seems we may make
  only one major release.

- Improves information about `protobuf` support for versions before v28,
  Scala 2.11, and the upcoming Bzlmod `compatibility_level` setting.

- In `scala_config.bzl`, changes the private `_default_scala_version()`
  to the public `DEFAULT_SCALA_VERSION`.

- Adds `allow_empty = True` to a `glob` expression in
  `//test/semanticdb:lib_with_tempsrc`.

- Removes Scala 2.11 test cases from `test_thirdparty_version.sh` and
  `test_version.sh`.

This change is smaller and more focused than bazelbuild#1710, and should
ultimately make that pull request smaller and/or easier to review.

The motivations for the individual changes are:

- The public `DEFAULT_SCALA_VERSION` constant makes this value
  accessible to the upcoming module extension.

- `glob` requires an explicit `allow_empty = True` parameter in Bazel 8,
  in which `--incompatible_disallow_empty_glob` defaults to `True`.

- ScalaPB 0.9.8, the last version compatible with Scala 2.11, does not
  support `protobuf` v25.6 or later. For this reason, we must remove the
  Scala 2.11 test cases, as documented in the `README.md` updates. See
  also bazelbuild#1712.

We should consider dropping Scala 2.11 support at this point, since
there's no ScalaPB release for it that supports later versions of
`protobuf`. That, and we could remove some of the special case code
added in the following changes, amongst other 2.11 support details:

- bazelbuild#1631
- bazelbuild#1648
- bazelbuild#1687
- bazelbuild#1688