docs: reset the changelog after the v1.2.1 release [no ci]

[TheGupta2012]

Summary

v1.2.1 shipped on 23 September 2026. This resets ## Unreleased to hold only unshipped work, and archives the release.

What changed

• Removed the five entries reproduced in the v1.2.1 release notes: opaque declarations fail to parse, blocking Quantinuum hqslib1 and any qasm2 file using them #370, loads() does not resolve custom include statements, unlike load() #368 and feat: add compact_gate_arguments module setting to the printer #427 under Added, and both ci: cancel superseded runs and trim the PR wheel matrix #419 entries under Other.
• Added v1.2.1 to the top of Past Release Notes.

Net: CHANGELOG.md, +1 / -5. No other file is touched.

What was deliberately kept

Two entries merged after the v1.2.1 tag and are not in the release. They stay in Unreleased and belong to the next release:

Entry	Section	PR
OpenQASM 3 end; statement support	Added	#424
Clifford+T exact rotation fix	Fixed	#429

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[TheGupta2012]

Merging since no updates to code , only changelog