Add release header when updating changelog (#10794)

The changelog update script now adds a release header if it doesn't
find one already that matches the current release candidate version.

development/auto-changelog.js

@@ -80,17 +80,24 @@ async function main() {
       `${versionHeader}$|${versionHeader}\\s`
     : currentDevelopBranchHeader;
 
-  const releaseHeaderIndex = changelogLines.findIndex((line) =>
+  let releaseHeaderIndex = changelogLines.findIndex((line) =>
     line.match(new RegExp(currentReleaseHeaderPattern, 'u')),
   );
   if (releaseHeaderIndex === -1) {
+    if (!isReleaseCandidate) {
       throw new Error(
-      `Failed to find release header '${
-        isReleaseCandidate ? versionHeader : currentDevelopBranchHeader
-      }'`,
+        `Failed to find release header '${currentDevelopBranchHeader}'`,
       );
     }
 
+    // Add release header if not found
+    const firstReleaseHeaderIndex = changelogLines.findIndex((line) =>
+      line.match(/## \d+\.\d+\.\d+/u),
+    );
+    changelogLines.splice(firstReleaseHeaderIndex, 0, versionHeader, '');
+    releaseHeaderIndex = firstReleaseHeaderIndex;
+  }
+
   const prNumbersWithChangelogEntries = [];
   for (const line of changelogLines) {
     const matchResults = line.match(/- \[#(\d+)\]/u);