[Ops] Fix deployment/project purging logic (elastic#172082)

## Summary
We've noticed that on certain PRs, the deployments are cleaned up right
after they're creating, with the indication that it was stale.
(elastic#170579 - cleaned up in
https://buildkite.com/elastic/kibana-purge-cloud-deployments/builds/15440#018c1605-144b-4db5-b29a-e3af3c9b1c70)

Turns out, the logic that acquires the last commit's timestamp is
relying on the `gh pr` api, and commits are limited there to 100. It's
easier to look at the pr's `updatedAt` value.

- change purge logic to be based on the PRs updatedAt label instead of
the max-100th commit's timestamp

See: https://elastic.slack.com/archives/C5UDAFZQU/p1701093461763989

.buildkite/scripts/steps/cloud/purge_deployments.ts

@@ -23,12 +23,11 @@ const DAY_IN_SECONDS = 60 * 60 * 24;
 for (const deployment of prDeployments) {
   try {
     const prNumber = deployment.name.match(/^kibana-pr-([0-9]+)$/)[1];
-    const prJson = execSync(`gh pr view '${prNumber}' --json state,labels,commits`).toString();
+    const prJson = execSync(`gh pr view '${prNumber}' --json state,labels,updatedAt`).toString();
     const pullRequest = JSON.parse(prJson);
     const prOpen = pullRequest.state === 'OPEN';
 
-    const lastCommit = pullRequest.commits.slice(-1)[0];
-    const lastCommitTimestamp = new Date(lastCommit.committedDate).getTime() / 1000;
+    const lastCommitTimestamp = new Date(pullRequest.updatedAt).getTime() / 1000;
 
     const persistDeployment = Boolean(
       pullRequest.labels.filter((label: any) => label.name === 'ci:cloud-persist-deployment').length

.buildkite/scripts/steps/cloud/purge_projects.ts

@@ -66,12 +66,11 @@ async function purgeProjects() {
     const NOW = new Date().getTime() / 1000;
     const DAY_IN_SECONDS = 60 * 60 * 24;
     const prJson = execSync(
-      `gh pr view '${project.prNumber}' --json state,labels,commits`
+      `gh pr view '${project.prNumber}' --json state,labels,updatedAt`
     ).toString();
     const pullRequest = JSON.parse(prJson);
     const prOpen = pullRequest.state === 'OPEN';
-    const lastCommit = pullRequest.commits.slice(-1)[0];
-    const lastCommitTimestamp = new Date(lastCommit.committedDate).getTime() / 1000;
+    const lastCommitTimestamp = new Date(pullRequest.updatedAt).getTime() / 1000;
 
     const persistDeployment = Boolean(
       pullRequest.labels.filter((label: any) => label.name === 'ci:project-persist-deployment')