Replace #22117. Implement it in a more maintainable way.
Some licenses have placeholders e.g. the BSD licenses start with this
line:
```
Copyright (c) <year> <owner>.
```
This PR replaces the placeholders with the correct value when initialize
a new repo.
### FAQ
- Why not use a regex?
It will be a pretty complicated regex which could be hard to maintain.
- There're still missing placeholders.
There are over 500 licenses, it's impossible for anyone to inspect all
of them alone. Please help to add them if you find any, and it is also
OK to leave them for the future.
---------
Co-authored-by: Giteabot <teabot@gitea.io>
