Update documentation and --help output to properly indicate supported puppet gem version format

[isaiahfrantz]

Describe the bug
The pdk docs (in many locations but 2 are below) indicate that you can provide a full puppet version but it appears that you only support the latest minor release of each major release.
https://www.puppet.com/docs/pdk/3.x/pdk_reference#pdk-validate-command
https://www.puppet.com/docs/pdk/3.x/pdk_testing#validate-a-module

This error message is misleading, causing people to waste time trying to determine what the issue is:

❯ pdk validate --puppet-version=7.24.0
pdk (ERROR): Unable to find a Puppet gem matching ~> 7.24.0.

I finally found out from folks on the puppet testing slack channel that pdk only supports the major:

❯ pdk validate --puppet-version=7
pdk (INFO): Using Ruby 2.7.8
pdk (INFO): Using Puppet 7.27.0
pdk (INFO): Running all available validators...

This uses to work as expected with pdk v1.x, (ie you could get and test against any version of puppet that there was a gem for). I have been running into this error since v2 came out, as have other people considering the results from a google search.

In addition to fixing the docs to clearly indicate that you can only ask for --puppet-version=7 or --puppet-version=8, please change the error from:

pdk (ERROR): Unable to find a Puppet gem matching ~> 7.24.0.

to something immediately meaningful to the developer, like:

pdk (ERROR): We only support testing against the latest minor release for each major Puppet gem release, please try %{requirement.split('.').first} instead.