Fallback to RubyGems 3.4.22 for Ruby 2.7

```
$ ruby -v
ruby 2.7.8p225 (2023-03-30 revision 1f4d455848) [x86_64-linux]
$ gem update --system
Updating rubygems-update
ERROR:  Error installing rubygems-update:
        rubygems-update-3.5.6 requires Ruby version >= 3.0.0. The current ruby version is 2.7.8.
$
```

https://github.com/rubygems/rubygems/blob/master/CHANGELOG.md#350--2023-12-15
> Drop ruby 2.6 and 2.7 support. Pull request #7116 by deivid-rodriguez

ruby/rubygems#7116

Author: yahonda

.github/workflows/test.yml

@@ -76,7 +76,7 @@ jobs:
           ./ci/setup_accounts.sh
       - name: Update RubyGems
         run: |
-          gem update --system
+          gem update --system || gem update --system 3.4.22
       - name: Bundle install
         run: |
           bundle install --jobs 4 --retry 3