fix(docs-infra): correctly check PR files on preview server (#25671)

According to the docs, the response of GitHub's [PR files API][1]
_"includes a maximum of 300 files"_. This means that if a PR contains
more files, it is possible that not all files are retrieved (which
could, for example, give a false negative for the "significant files
touched" check - not likely but possible).

This commit fixes it by using paginated requests to retrieve all changed
files.

[1]: https://developer.github.com/v3/pulls/#list-pull-requests-files

PR Close #25671

aio/aio-builds-setup/dockerbuild/scripts-js/lib/common/github-pull-requests.ts

@@ -74,6 +74,6 @@ export class GithubPullRequests {
    */
   public fetchFiles(pr: number): Promise<FileInfo[]> {
     assert(pr > 0, `Invalid PR number: ${pr}`);
-    return this.api.get<FileInfo[]>(`/repos/${this.repoSlug}/pulls/${pr}/files`);
+    return this.api.getPaginated<FileInfo>(`/repos/${this.repoSlug}/pulls/${pr}/files`);
   }
 }