5.0.28 still supports writing mixed schema buckets to timeseries collection, so we can pin this version and retain test coverage for upgrading versions in the presence of mixed schema buckets.

Affected tests:

• jstests/multiVersion/genericBinVersion/timeseries_collection_mixed_type.js
• jstests/multiVersion/targetedTestsLastLtsFeatures/upgrade_downgrade_timeseries_collection_from_last_lts.js
• jstests/multiVersion/targetedTestsLastLtsFeatures/timeseries_collection_mixed_schema_index_build_stepdown.js