feat(release): generate the GitHub release body from the changelog (#2987)

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

cmd/release/notes.go

@@ -0,0 +1,48 @@
+package main
+
+import (
+	"fmt"
+	"io"
+	"os"
+	"strings"
+
+	"github.com/Masterminds/semver/v3"
+)
+
+func notes(w io.Writer) error {
+	version, err := getVersion(versionFile)
+	if err != nil {
+		return err
+	}
+
+	b, err := os.ReadFile(changelogSource)
+	if err != nil {
+		return err
+	}
+
+	section := changelogSection(string(b), version)
+	if section == "" {
+		return nil
+	}
+	_, err = fmt.Fprintln(w, section)
+	return err
+}
+
+func changelogSection(changelog string, version *semver.Version) string {
+	heading := fmt.Sprintf("## v%s ", version)
+	var section []string
+	var found bool
+	for line := range strings.SplitSeq(changelog, "\n") {
+		if strings.HasPrefix(line, "## ") {
+			if found {
+				break
+			}
+			found = strings.HasPrefix(line, heading)
+			continue
+		}
+		if found {
+			section = append(section, line)
+		}
+	}
+	return strings.TrimSpace(strings.Join(section, "\n"))
+}