ci: rollback dual release line scheme

[sebastiancorrea81]

Summary

Reverts the temporary dual release line CI scheme (package/* → 7.x, resto → 6.x), per the runbook at docs/ci/dual-release-line-rollback.md.

• Revert "fix(ci): correct release-please target-branch resolution for the dual release line scheme (fix(ci): correct release-please target-branch resolution for the dual release line scheme #504)"
• Revert "ci: temporary dual release line (package -> 7.x, resto -> 6.x) (ci: temporary dual release line (package -> 7.x, resto -> 6.x) #498)"

The rollback doc itself is removed as part of this PR (the scheme it documented no longer exists). Companion PR against main: #552.

Test plan

• git diff against pre-scheme state confirmed clean (matches the fallback manual in the runbook)
• Same result verified identical between this branch and the companion main rollback branch

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