[Fleet] Fail build on unable to retrieve agent versions list (#154110)

## Summary

Resolves #153923

This PR:

1. [Fails **non-PR**
builds](https://buildkite.com/elastic/kibana-pull-request/builds/116952#01873497-c5a2-4031-9f5b-84df00e8368b)
if we are unable to retrieve the list of Elastic Agent versions from the
website API. If a build ships without this list being retrieved, users
will not have the correct version choices to upgrade their agents to
(see above issue)
2. Updates the API endpoint used from
`https://www.elastic.co/api/product_versions` to
`https://www.elastic.co/content/product_versions`, which appears to be a
more stable version. The `/api` one is currently down:
elastic/website-development#10820

src/dev/build/tasks/fetch_agent_versions_list.ts

@@ -19,22 +19,29 @@ const getAvailableVersions = async (log: ToolingLog) => {
   };
   // Endpoint maintained by the web-team and hosted on the elastic website
   // See https://github.com/elastic/website-development/issues/9331
-  const url = 'https://www.elastic.co/api/product_versions';
-  try {
-    log.info('Fetching Elastic Agent versions list');
-    const results = await fetch(url, options);
+  const url = 'https://www.elastic.co/content/product_versions';
+  log.info('Fetching Elastic Agent versions list');
+  const results = await fetch(url, options);
+  const rawBody = await results.text();
 
-    const jsonBody = await results.json();
+  try {
+    const jsonBody = JSON.parse(rawBody);
 
     const versions: string[] = (jsonBody.length ? jsonBody[0] : [])
       .filter((item: any) => item?.title?.includes('Elastic Agent'))
       .map((item: any) => item?.version_number);
 
-    log.info(`Retrieved available versions`);
+    log.info(`Retrieved available Elastic Agent versions`);
     return versions;
   } catch (error) {
-    log.warning(`Failed to fetch versions list`);
-    log.warning(error);
+    log.warning(`Failed to fetch Elastic Agent versions list`);
+    log.info(`Status: ${results.status}`);
+    log.info(rawBody);
+    if (process.env.BUILDKITE_PULL_REQUEST === 'true') {
+      log.warning(error);
+    } else {
+      throw new Error(error);
+    }
   }
   return [];
 };
@@ -47,8 +54,8 @@ export const FetchAgentVersionsList: Task = {
     const versionsList = await getAvailableVersions(log);
     const AGENT_VERSION_BUILD_FILE = 'x-pack/plugins/fleet/target/agent_versions_list.json';
 
-    if (versionsList !== []) {
-      log.info(`Writing versions list to ${AGENT_VERSION_BUILD_FILE}`);
+    if (versionsList.length !== 0) {
+      log.info(`Writing Elastic Agent versions list to ${AGENT_VERSION_BUILD_FILE}`);
       await write(
         build.resolvePath(AGENT_VERSION_BUILD_FILE),
         JSON.stringify(versionsList, null, '  ')