Summary

Updates docs/development/release-create.md with lessons learned during the 9.1.0-RC1 release process.

• Version string format (new section): RC filenames must use lowercase rc with a hyphen (e.g., 9.1.0-rc1). The Configurator firmware flasher uses a case-sensitive regex; uppercase RC or underscore separators cause the target board name to be misread, making the firmware invisible in the flasher even after a successful upload.
• Fix rename script: The output filename used _rc2_ (underscore) but the required format is -rc2_ (hyphen). This would have produced filenames that the Configurator cannot parse.
• Fix configurator artifact download: The previous instructions used find . -mindepth 2 -type f -exec mv -t . {} + to flatten all files into one directory. This exact pattern caused a Windows .exe to be packaged inside a macOS DMG in the 9.0.0 release. Replaced with explicit per-platform mkdir/mv.
• PR branch verification (changelog section): gh pr list returns PRs by merge date regardless of target branch, so PRs merged to maintenance-10.x appear in results even if they are not part of the current release. Added commands to verify each PR is actually on the release branch.
• gh release create examples: Added --prerelease for RC releases and --target <commit-sha> to tag a specific commit atomically without needing local repo access.
• Post-publish verification: Added a step to verify firmware appears in the Configurator Firmware Flasher tab (with "Show unstable releases" enabled) before publishing the Configurator release. This is the end-to-end validation that the filename format and release structure are correct.

Test plan

• Read through the updated document for consistency
• Verify the rename script example produces inav_9.1.0-rc1_TARGET.hex format
• Confirm no other references to the old uppercase RC naming remain