[CI][Python] Revert pinning miniforge once mamba solver issue is resolved

[adamreeve]

See #49998 for context.

Once mamba-org/mamba#4274 is fixed and released in miniforge, or we drop Python 3.10 from CI (it's EOL on 2026-10-31), we should revert #49999.

[adamreeve]

The mamba fix is released in version 2.7.0, but the latest miniforge has pinned the mamba version to 2.5.0. That's before the performance regression, but I think we should wait until they use mamba >=2.7.0 before reverting the version pinning.