CI: jruby-head could install a version of RubyGems supported

Example error in jruby-head:

```
Run gem update --system 2.7.11
ERROR:  rubygems 2.7.11 is not supported on 3.1.0. The oldest version supported by this ruby is 3.2.3
Error: Process completed with exit code 1.
```

Seen in https://github.com/jruby/warbler/pull/511