Meta: update repository files

See https://github.com/whatwg/spec-factory for details.

PULL_REQUEST_TEMPLATE.md

@@ -8,10 +8,11 @@ Thank you for contributing to the HTML Standard! Please describe the change you
 - [ ] [Tests](https://github.com/web-platform-tests/wpt) are written and can be reviewed and commented upon at:
    * …
 - [ ] [Implementation bugs](https://github.com/whatwg/meta/blob/main/MAINTAINERS.md#handling-pull-requests) are filed:
-   * Chrome: …
-   * Firefox: …
-   * Safari: …
+   * Chromium: …
+   * Gecko: …
+   * WebKit: …
    * Deno (only for timers, structured clone, base64 utils, channel messaging, module resolution, web workers, and web storage): …
    * Node.js (only for timers, structured clone, base64 utils, channel messaging, and module resolution): …
+- [ ] [MDN issue](https://github.com/whatwg/meta/blob/main/MAINTAINERS.md#handling-pull-requests) is filed: …
 
 (See [WHATWG Working Mode: Changes](https://whatwg.org/working-mode#changes) for more details.)