bump MSRV to 1.75.0 for all crates

[lalitb]

Changes

As discussed in the community meeting (ref - #2374 (comment)) , opening this PR to bump the MSRV from 1.70.0/1.71.1 to 1.75.0 for all crates.

If there are any concerns or objections to increasing the MSRV, please share your feedback in this PR.

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 79.3%. Comparing base (d0ef365) to head (0d1aefc).
Report is 2 commits behind head on main.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #2417   +/-   ##
=====================================
  Coverage   79.3%   79.3%           
=====================================
  Files        122     122           
  Lines      21569   21569           
=====================================
  Hits       17114   17114           
  Misses      4455    4455           

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[cijothomas]

LGTM.
https://releases.rs/docs/1.75.0/ was release an year ago, and is much more liberal than the official policy https://github.com/open-telemetry/opentelemetry-rust?tab=readme-ov-file#supported-rust-versions