docs: prepare 1.5.0 release notes

[adlio]

Summary

• add the 1.5.0 release notes entry to README so the automated release workflow can publish curated notes
• document that Go 1.25.5+ is now required because the dockertest v4 migration raised the minimum supported toolchain
• capture the security-driven Docker test dependency upgrade and recent CI/release automation improvements

Test Plan

• go test ./...
• python3 README extraction script for 1.5.0

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.79%. Comparing base (61dd988) to head (321667b).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #50   +/-   ##
=======================================
  Coverage   99.79%   99.79%           
=======================================
  Files          10       10           
  Lines         483      483           
=======================================
  Hits          482      482           
  Misses          1        1           

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