Protect released changelog in CI

[XSAM]

resolve #1682

This PR adds a new action in CI to protect released sections in the changelog from being changed. It compares the released sections in the changelog from the base branch to the current PR. It fails if a change happens.

Apply "Unlock Released Changelog" to bypass this check when doing a new release.

Action result if we changed the unreleased section: https://github.com/open-telemetry/opentelemetry-go/pull/5560/commits/d640deb435cb54802c6819140ee467ad67b068df Action result if we changed the released section: https://github.com/open-telemetry/opentelemetry-go/pull/5560/commits/d672de7a54d97979b4ec83f864f2c1789f6b292a

---

Meaning of awk '/^\<!-- Released section --\>/ {flag=1} /^\<!-- Released section ended --\>/ {flag=0} flag'

• /^\<!-- Released section --\>/ {flag=1}
  • Matches lines starting with <!-- Released section -->.
  • Sets the flag variable to 1 (starting the printing of lines).
• /^\<!-- Released section ended --\>/ {flag=0}
  • Matches lines starting with <!-- Released section ended -->.
  • Sets the flag variable to 0.
  • So it is ok to change external reference links, like

• flag lines are printed only when the flag is 1.

[dmathieu]

This is a great idea!