fix: ignore trailing slash in GH CVSS strings

[willmurphyscode]

Sometimes these strings have a trailing slash; ignore the trailing slash rather than throwing away the entire CVSS info because of it.

I think that an otherwise well-formed CVSS that happens to have a / on the end shouldn't be thrown away just because of the /.

Previously, there were a lot of log entries like this:

[ERROR] error transforming CVSS vector CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:H/I:H/A:N/, skipping it for GHSA-gr4j-r575-g665
Traceback (most recent call last):
  File "/home/runner/work/vunnel/vunnel/src/vunnel/providers/github/parser.py", line 551, in _make_cvss
    cvss3_obj = CVSS3(cvss_vector)
  File "/home/runner/.virtualenvs/vunnel-A95CY2kd-py3.10/lib/python3.10/site-packages/cvss/cvss3.py", line 114, in __init__
    self.parse_vector()
  File "/home/runner/.virtualenvs/vunnel-A95CY2kd-py3.10/lib/python3.10/site-packages/cvss/cvss3.py", line 133, in parse_vector