docs: Remove Gitter (#2678)

Remove Gitter (which is now hosted on Matrix) since it's been abandoned for way too long

.github/ISSUE_TEMPLATE/config.yml

@@ -3,12 +3,6 @@ contact_links:
   - name: Start a discussion
     url: https://github.com/nektos/act/discussions/new
     about: You can ask for help here!
-  - name: Ask on Gitter
-    url: https://gitter.im/nektos/act
-    about: You can ask for help here!
-  - name: Ask on Gitter (via Matrix)
-    url: https://matrix.to/#/#nektos_act:gitter.im?via=gitter.im&via=matrix.org
-    about: If you prefer Matrix over Gitter
   - name: Want to contribute to act?
     url: https://github.com/nektos/act/blob/master/CONTRIBUTING.md
     about: Be sure to read contributing guidelines!