fix(scripts/releaser): fix tag sorting and changelog blurb for older branches (#24798)

Fixes two bugs in the release tool.

## 1. RC tags chosen over release tags on release branches

`allSemverTags()` and `mergedSemverTags()` rely on `git tag
--sort=-v:refname` for ordering. Git's version sort treats pre-release
suffixes (e.g. `-rc.0`) as *greater* than the base release version,
which is the opposite of semver where `v2.32.0 > v2.32.0-rc.0`.

When the release branch code iterates the tag list looking for the first
matching `major.minor`, it finds the RC tag first, leading to incorrect
version suggestions (e.g. suggesting `v2.32.0` again instead of
`v2.32.1`).

**Fix:** Re-sort parsed tags using the existing `GreaterThan` method via
a new `sortVersionsDesc` helper.

## 2. Misleading mainline changelog blurb on ESR/older branch patches

When releasing a patch on an older branch (e.g. `release/2.29` for ESR),
the version is neither mainline nor stable. Declining the stable prompt
would always produce the mainline changelog note ("This is a mainline
Coder release..."), which is incorrect.

**Fix:** Only emit the mainline note when the version's minor matches
the current mainline series. For older branches the changelog omits the
note entirely.

> Generated by Coder Agents

scripts/releaser/version.go

@@ -3,6 +3,7 @@ package main
 import (
 	"fmt"
 	"regexp"
+	"sort"
 	"strconv"
 	"strings"
 )
@@ -85,6 +86,17 @@ func (v version) Equal(b version) bool {
 	return v.Major == b.Major && v.Minor == b.Minor && v.Patch == b.Patch && v.Pre == b.Pre
 }
 
+// sortVersionsDesc sorts a slice of versions in descending order
+// using semver-correct comparison. This is necessary because git's
+// --sort=-v:refname treats pre-release suffixes (e.g. -rc.0) as
+// greater than the release version, which is the opposite of semver
+// where v2.32.0 > v2.32.0-rc.0.
+func sortVersionsDesc(tags []version) {
+	sort.Slice(tags, func(i, j int) bool {
+		return tags[i].GreaterThan(tags[j])
+	})
+}
+
 // allSemverTags returns all semver tags sorted descending.
 func allSemverTags() ([]version, error) {
 	out, err := gitOutput("tag", "--sort=-v:refname")
@@ -100,6 +112,7 @@ func allSemverTags() ([]version, error) {
 			tags = append(tags, v)
 		}
 	}
+	sortVersionsDesc(tags)
 	return tags, nil
 }
 
@@ -119,5 +132,6 @@ func mergedSemverTags() ([]version, error) {
 			tags = append(tags, v)
 		}
 	}
+	sortVersionsDesc(tags)
 	return tags, nil
 }