Removing the default macOS toolchain from rules_cc

## Overview

Currently for historical reasons there are 2 toolchains on the bazelbuild org that can target macOS. One in [apple_support](https://github.com/bazelbuild/apple_support/blob/15885c37058df85045f349a9576d9020f2b239f3/crosstool/cc_toolchain_config.bzl) (that also supports iOS and other Apple platforms), and the one in [rules_cc](https://github.com/bazelbuild/rules_cc/blob/f65555ccb4257d65e0a5379f9f476dd748203d4b/cc/private/toolchain/unix_cc_toolchain_config.bzl) that is roughly shared with the default linux toolchain (roughly because many things differ between the 2 platforms).

Before this toolchain was moved out of the bazel repo and into `rules_cc`, it was not feasible to remove it. Now that both exist in separate repos, and both of these repos are depended on by [bazel by default](https://github.com/bazelbuild/bazel/blob/b1eed3b9a51e86b605f58b3182ab97feb2f1a939/src/MODULE.tools#L39-L42), we are planning to remove the duplicated `rules_cc` version of this toolchain, and require users to use the toolchain from `apple_support` moving forward.

## Why

There are a few reasons for this:

- the toolchain in `rules_cc` isn't hermetic because of complexities with Apple's tools, this has been solved in `apple_support`, and to solve it here would just require duplicating the same implementation
- this reduces confusion from users because both toolchains are always registered, and always conflict, so which toolchain you actually get depends on the order of `bazel_dep` targets in your `MODULE.bazel`
- this reduces maintenance overhead for the maintainers
- Previously the `apple_support` toolchain did not support the Xcode Command Line Tools only (meaning it required a full Xcode install) but this is no longer the case

## Plan

1. Merge an **off by default** environment variable to disable the `rules_cc` toolchain for early testing with `--repo_env=BAZEL_USE_LEGACY_MACOS_TOOLCHAIN=0`, release it ASAP (hopefully 0.2.20)
2. Flip the default, while still allowing users to bring the old toolchain back with `--repo_env=BAZEL_USE_LEGACY_MACOS_TOOLCHAIN=1`, release it 1 release later (hopefully 0.2.21)
3. Field issues that come up from subtleties for a few months
4. Fully delete the `rules_cc` toolchain, probably after bazel 10.x releases depending on the number of issues that come up

## Known changes

- The `apple_support` toolchain has slightly different arguments / ordering of arguments than `rules_cc` that will not be aligned and likely won't cause issues but could, we'll have to field these case by case
- Projects can be calling `configure_unix_toolchain` manually with their own tools and expect the resulting toolchain to work with macOS. It's possible we could expand the `apple_support` toolchain to support similar behavior, but this will be considered case by case
- `toolchains_llvm` does the above, we will have to decide how to handle that repo
- `apple_support` only supports compilers shipped with Apple's tools, not the `brew` install of `clang` or `gcc`, support for this is possible some please comment if this is something that is desired (although possibly a better recommendation would be to move to one of the fully hermetic toolchains instead)

## Other notes

- If you're using bazel 7.x or earlier, even if you update `rules_cc` this likely doesn't apply since this toolchain still lived in bazel at the time. This would apply to 8.x+
- `apple_support` doesn't support WORKSPACE anymore, but it could again if needed. The last version with workspace support was [1.24.5](https://github.com/bazelbuild/apple_support/releases/tag/1.24.5) which could likely also be used instead

## Feedback

Please comment here, or file issues on [`apple_support`](https://github.com/bazelbuild/apple_support/) with any issues you see with this plan, or incompatibilities you hit with this change.

If you would like to test this today you can add `apple_support` to your `MODULE.bazel` above `rules_cc` like:

```bzl
bazel_dep(name = "apple_support", version = "2.6.1")
bazel_dep(name = "rules_cc", version = "0.2.19")
```

Start by reading cc/private/toolchain/unix_cc_toolchain_config.bzl in rules_cc and the linked apple_support crosstool/cc_toolchain_config.bzl, then review the MODULE.bazel dependency ordering described in the issue. The work is complete only after the legacy macOS toolchain is removed without breaking supported macOS users, with compatibility issues from the listed migration cases addressed.

Written by the indexing model from the issue text.