Bump minimum ruby version to 3.1

[DuncSmith]

Dependabot uses ruby v3.0 to bundle, however nokogiri requires ruby v3.1, resulting in dependabot to fail

Dependabot can't resolve your Ruby dependency files
Dependabot failed to update your dependencies because there was an error resolving your Ruby dependency files.

Dependabot encountered the following error:

Could not find compatible versions

Because nokogiri >= 1.18.3 depends on Ruby >= 3.1.0, < 3.5.dev
  and nokogiri = 1.18.3 depends on Ruby >= 3.1.0,
  nokogiri >= 1.18.3 requires Ruby >= 3.1.0.
So, because Gemfile depends on nokogiri = 1.18.3
  and current Ruby version is = 3.0.6,
  version solving has failed.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[NikolasPilavakis]

Thanks @DuncSmith - Since this is a public gem, would it make sense to cut a new version?