semver is not handled in generate_units_documentation
### Summary
Since Sylva 1.5.0, if a GitRepository has a semver, `check_versions_downgrade` script will fail
### Steps to reproduce
1. In `values.yaml`, define a `source_templates` like this:
```
enablers-bootstrap:
kind: GitRepository
spec:
url: https://gitlab.tech.orange/caascnp/enablers-bootstrap.git
ref:
semver: ">=0.1.0 <0.2.0" # allow changes during the build phase
```
2. Execute `check_versions_downgrade` script
Will result in this CI logs :
```
unit enablers-bootstrap / repo enablers-bootstrap, spec.ref has neither tag, commit or branch (??)
```
### What is the current *bug* behavior?
`semver` is authorized in the schema, but is not fully supported in this case : it raises an exception here : https://gitlab.com/sylva-projects/sylva-core/-/blob/1.5.0/tools/generate_units_documentation.py?ref_type=tags#L113
### What is the expected *correct* behavior?
Not 100% sure, as the goal of the calling function is to get the real version, which is not simple in case of a semver.
issue