Add current NetBox version in the `PluginConfig.validate()` error output

[markkuleinio]

NetBox version

v4.0.1

Feature type

New functionality

Proposed functionality

When PluginConfig.validate() determines that the current NetBox version does not meet the plugin requirements, also print out the current NetBox version in the exception message.

Use case

Currently the error message only prints the version that the plugin mandates, but due to possible installation directory confusion it would be good to also print the current NetBox version. Example case: #16088

Database changes

None

External dependencies

None

[markkuleinio]

If this is accepted, I can send the PR.