feat: add release candidate (RC) support to release tooling (#23600)

This adds full RC release support to the release scripts and GitHub
Actions workflow. Previously, the tooling only supported stable and
mainline releases with strict vMAJOR.MINOR.PATCH semver tags.

Changes:
- scripts/releaser/version.go: Add Pre field to version struct for
prerelease suffixes (e.g. "rc.0"), update regex, parsing, String(),
comparison methods, and add IsRC()/rcNumber() helpers.
- scripts/releaser/release.go: Detect RC branches (release/X.Y-rc.N),
suggest RC version numbers, auto-set "rc" channel (skipping
stable/mainline prompt), add RC advisory to release notes, skip docs
update for RC releases.
- .github/workflows/release.yaml: Add "rc" channel option, fix branch
derivation for RC tags (v2.32.0-rc.0 -> release/2.32-rc.0 instead of
broken release/2.32.0-rc), skip homebrew/winget/package publishing for
RC releases.
- scripts/release/publish.sh: Add --rc flag, pass --prerelease to gh
release create for RC releases.
- scripts/releaser/version_test.go: Add comprehensive unit tests for
version parsing, string formatting, IsRC, rcNumber, GreaterThan, and
Equal with RC versions.

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scripts/release/publish.sh

@@ -34,11 +34,12 @@ if [[ "${CI:-}" == "" ]]; then
 fi
 
 stable=0
+rc=0
 version=""
 release_notes_file=""
 dry_run=0
 
-args="$(getopt -o "" -l stable,version:,release-notes-file:,dry-run -- "$@")"
+args="$(getopt -o "" -l stable,rc,version:,release-notes-file:,dry-run -- "$@")"
 eval set -- "$args"
 while true; do
 	case "$1" in
@@ -46,6 +47,10 @@ while true; do
 		stable=1
 		shift
 		;;
+	--rc)
+		rc=1
+		shift
+		;;
 	--version)
 		version="$2"
 		shift 2
@@ -68,6 +73,10 @@ while true; do
 	esac
 done
 
+if [[ "$stable" == 1 ]] && [[ "$rc" == 1 ]]; then
+	error "Cannot specify both --stable and --rc"
+fi
+
 # Check dependencies
 dependencies gh
 
@@ -162,6 +171,11 @@ if [[ "$stable" == 1 ]]; then
 	latest=true
 fi
 
+prerelease_flag=()
+if [[ "$rc" == 1 ]]; then
+	prerelease_flag=(--prerelease)
+fi
+
 target_commitish=main # This is the default.
 # Skip during dry-runs
 if [[ "$dry_run" == 0 ]]; then
@@ -176,6 +190,7 @@ fi
 true |
 	maybedryrun "$dry_run" gh release create \
 		--latest="$latest" \
+		"${prerelease_flag[@]}" \
 		--title "$new_tag" \
 		--target "$target_commitish" \
 		--notes-file "$release_notes_file" \