Add a bump implementation for GentooVersion

bump is required to handle revision version ranges correctly for GentooImporterPipeline

see:
https://github.com/aboutcode-org/vulnerablecode/pull/2090#discussion_r2772936113

Expected:
```
>>> from univers.versions import GentooVersion
>>> GentooVersion("2.23.3").bump()
GentooVersion(string='2.24')
```

Actual:
```
>>> GentooVersion("2.23.3").bump()
Traceback (most recent call last):
File "", line 1, in
AttributeError: 'GentooVersion' object has no attribute 'bump'
```