Renames the two arm64-backed Evergreen build variants so their names reflect their actual architecture:

• `linux-64-debug-required` → `linux-arm64-debug-required`
• `linux-64-debug-disagg-required` → `linux-arm64-debug-disagg-required`

*Motivation*

Both variants run exclusively on `amazon2023-arm64` / `rhel8.8-arm64` distros, so the existing `linux-64` prefix is misleading. Adding the `arm` prefix makes the architecture explicit and consistent with the arm64 naming used elsewhere in the config.

*Changes*

• `etc/evergreen_yml_components/variants/sanitizer/test_dev.yml`: rename both variant definitions, including the `&linux-…-disagg-required` YAML anchor and its `compile_variant: *…` alias reference.
• `etc/evergreen_timeouts.yml`: rename the `linux-64-debug-required` override key.
• `bazel/resmoke/download_historic_runtimes.py`: update `DEFAULT_BUILD_VARIANT`.
• `buildscripts/evergreen_resmoke_job_count.py`: update the variant name in the `maybe_override_num_jobs_on_required` allowlist tuple.

*Scope notes*

• The sibling `linux-64-debug-required-toolchain-v5` variant is intentionally left unchanged (out of scope).
• Renaming an Evergreen build variant resets its history/reliability under the new name; historical test-runtime data keyed by the old name will no longer match. This is expected for a variant rename and worth confirming with CI/DevProd before merging.

*Testing*

Ran `bazel run format` (no formatting changes produced beyond the renames). Evergreen patch to validate the renamed variants will be linked in the comments.