Bump minimum required version to Ruby 2.5

On Platform 7 we will no longer support versions of Ruby that have gone end of life: 2.3 and 2.4.

To achieve this we need to do the following on the Puppet main stream:

• update the Puppet gemspec: https://github.com/puppetlabs/puppet/blob/main/.gemspec#L21
• update the Travis configuration: https://github.com/puppetlabs/puppet/blob/main/.travis.yml
• update project_data: https://github.com/puppetlabs/puppet/blob/main/ext/project_data.yaml#L17