Fix and improve repo visibility checkbox when FORCE_PRIVATE is on (#3786)
This was [implemented](https://github.com/gogs/gogs/commit/022820103d622dda74aa1c619ca46838a14b762a) almost 10 years ago in Gogs.
It was using `readonly` attribute instead of `disabled` on checkbox which was doing nothing. According to [MDN](https://developer.mozilla.org/en-US/docs/Web/HTML/Attributes/readonly), `readonly` attribute is not applicable to checkbox inputs. This bug was reported by mrwusel in Matrix room.
Fork page that has similar UI feature did not have this bug.
- replace `readonly` with `disabled`
- do not put info about the restriction directly into the checkbox title
### Before

### After

### In other case

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/3786
Reviewed-by: Earl Warren <earl-warren@noreply.codeberg.org>