Stabilizes the nightly code coverage variant instead of turning it off and letting it rot.

These tasks do not measure coverage — results are not uploaded anywhere while we are between vendors, so user-visible impact is minimal. What they do is keep the coverage workflow from breaking: that `--collect_code_coverage` still compiles, instrumented binaries still link and run, the lcov merge still produces a report, and the local `bazel coverage` workflow still works.

Changes:

• `bazel_coverage` now targets `base` + `platform` + `stdx` + `bson` (~329 instrumented `cc_library` targets) instead of `//src/mongo/...` (~1436, effectively a full-server build) — the source of the slowness and OOMs.
• Dropped the jstest/passthrough and streams tasks, the streams module/build flag, and the compile task from the `AL2023-arm64-coverage` variant; only unittests run under coverage now.
• Removed the `rhel-93-64-bit-coverage` variant — it duplicated the arm64 signal and doubled the failure surface.
• Clarified the `codecoverage` / `unittestcoverage` patch alias descriptions.
• Added `etc/evergreen_yml_components/variants/codecoverage/README.md` covering what these tasks are for, why the target list is what it is, gotchas, and the known follow-ups (coverage summary artifact, resmoke coverage path, gcov cleanup, expansions plumbing).

1. 1. 1. Testing
         Evergreen patch on the `codecoverage` alias.