DEV: Bump the limits on group request text fields

Users submitting requests to join groups were not receiving errors when
the character limit for the request was exceeded. This also affects the
UX when admin-created group request templates are inserted into the
request.

This patch bumps the limits.

- https://meta.discourse.org/t/group-membership-requests-suddenly-limited-to-274-characters/265127
- https://github.com/discourse/discourse/pull/19993

spec/requests/groups_controller_spec.rb

@@ -2208,7 +2208,7 @@ RSpec.describe GroupsController do
 
       expect(response.status).to eq(422)
       expect(response.parsed_body["errors"]).to contain_exactly(
-        "Reason is too long (maximum is 280 characters)",
+        "Reason is too long (maximum is 5000 characters)",
       )
     end