Merge all GF copyright checks

[simoncozens]

Description

fontbakery.checks.googlefonts.copyright is a good example of #4735 (many simple checks versus fewer larger checks).

Currently we have:

• A check to ensure the font name table copyright field has the right format
• A check to ensure the font name table copyright field is less than 500 characters
• A check to ensure the METADATA.pb copyright field has the right format
• A check to ensure the METADATA.pb copyright field is less than 500 characters
• A check to ensure the METADATA.pb copyright field and the name table copyright field are the same

This turns them into a single check

Checklist

• [ ] update CHANGELOG.md
• [ ] wait for the tests to pass
• [ ] request a review