chore: leverage fallback resolver for MSRV check

[gruebel]

Changes

• similar to chore: leverage fallback resolver for MSRV check opentelemetry-rust-contrib#263
• also updated check-external-types to 0.2.0

Merge requirement checklist

• CONTRIBUTING guidelines followed
• Unit tests added/updated (if applicable)
• Appropriate CHANGELOG.md files updated for non-trivial, user-facing changes
• Changes in public API reviewed (if applicable)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.3%. Comparing base (9a0099a) to head (7493403).

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #2976   +/-   ##
=====================================
  Coverage   81.3%   81.3%           
=====================================
  Files        126     126           
  Lines      24375   24375           
=====================================
  Hits       19827   19827           
  Misses      4548    4548           

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[hdost]

Same as comment on other repo