Update to upstream release 2.7.3
This MR updates to the latest Trusted QSL release from ARRL. It applies the following changes:
- Adds in missing upstream merge of 2.7.2.
- Fixes watch file to ignore non-existent PGP signature
- Imports TQSL 2.7.3
- Bumps to standards version 4.7.0
- Updates dates on copyright
- Fixes license declaration on AppStream metadata
- Updates changelog