Cleanup the momentjs dependency in @backstage/plugin-github-pull-requests-board

As per [ADR010: Use the Luxon Date Library](https://github.com/backstage/backstage/blob/master/docs/architecture-decisions/adr010-luxon-date-library.md),
we should replace momentjs with luxon.

Signed-off-by: Mengnan Gong <namco1992@gmail.com>

plugins/github-pull-requests-board/package.json

@@ -44,7 +44,7 @@
     "@material-ui/icons": "^4.9.1",
     "@material-ui/lab": "4.0.0-alpha.57",
     "@octokit/rest": "^19.0.3",
-    "moment": "^2.29.1",
+    "luxon": "^3.0.0",
     "react-use": "^17.2.4"
   },
   "devDependencies": {

plugins/github-pull-requests-board/src/utils/functions.ts

@@ -14,7 +14,7 @@
  * limitations under the License.
  */
 import { Entity } from '@backstage/catalog-model';
-import moment from 'moment';
+import { DateTime } from 'luxon';
 import {
   Reviews,
   PullRequests,
@@ -58,7 +58,7 @@ export const filterSameUser = (users: Author[]): Author[] => {
 };
 
 export const getElapsedTime = (start: string): string => {
-  return moment(start).fromNow();
+  return DateTime.fromISO(start).toRelative() ?? start;
 };
 
 export const formatPRsByReviewDecision = (