refactor(dev-infra): update octokit to latest version v18 (#42666)

We previously held off with updating Octokit to v18 due to
their more noticable issues with typings. This commit updates
us to the latest version in order to take advantage of the new
pagination API (which is also strongly-typed), and to not fall
behind too much over time (Octokit seems to change quite often..)

We work around the problem with the types for `getContent` by just
using a type cast with a TODO (and link to the issue). Similarly we
work around a problem where the Octokit types have an incorrect type
for the name of the labels array in an API response.

PR Close #42666

dev-infra/utils/git/github.ts

@@ -6,7 +6,10 @@
  * found in the LICENSE file at https://angular.io/license
  */
 
+import {OctokitOptions} from '@octokit/core/dist-types/types';
 import {graphql} from '@octokit/graphql';
+import {PaginateInterface} from '@octokit/plugin-paginate-rest';
+import {RestEndpointMethods} from '@octokit/plugin-rest-endpoint-methods/dist-types/generated/method-types';
 import {Octokit} from '@octokit/rest';
 import {RequestParameters} from '@octokit/types';
 import {query} from 'typed-graphqlify';
@@ -41,10 +44,14 @@ export class GithubClient {
   readonly repos = this._octokit.repos;
   readonly issues = this._octokit.issues;
   readonly git = this._octokit.git;
-  readonly paginate = this._octokit.paginate;
   readonly rateLimit = this._octokit.rateLimit;
 
-  constructor(private _octokitOptions?: Octokit.Options) {}
+  // Note: These are properties from `Octokit` that are brought in by optional plugins.
+  // TypeScript requires us to provide an explicit type for these.
+  readonly rest: RestEndpointMethods = this._octokit.rest;
+  readonly paginate: PaginateInterface = this._octokit.paginate;
+
+  constructor(private _octokitOptions?: OctokitOptions) {}
 }
 
 /**