Fix new issues found by `proselint`.
Also:
- silence this technical warning:
```
:0: DeprecationWarning: /home/runner/.proselintrc was found instead of a JSON file. Rename to /home/runner/.proselintrc.json.
```
- fix an input filename.
`proselints` fails now if an input file is missing.
Reported-by: Jay Satiro
Bug: https://github.com/curl/curl/pull/15291#issuecomment-2410505100Closes#15293
- they are mostly pointless in all major jurisdictions
- many big corporations and projects already don't use them
- saves us from pointless churn
- git keeps history for us
- the year range is kept in COPYING
checksrc is updated to allow non-year using copyright statements
Closes#10205
