Fix versioneer config to match version tags

[michaelosthege]

The config was slightly wrong: We're prefixing release tags with v not with rel-.

Closes #6303

Checklist

• Explain important implementation details 👆
• Make sure that the pre-commit linting/style checks pass.
• Link relevant issues (preferably in nice commit messages)
• Are the changes covered by tests and docstrings? A test would require the git history to get the previous tag, but we're just doing shallow checkouts in the CI pipeline. Deep checkout just for this isn't worth it..
• Fill out the short summary sections 👇

Major / Breaking Changes

• None

Bugfixes / New features

• Fixes a bug that excluded the latest release version from pm.__version__ in git installs.

Docs / Maintenance

• None

[codecov]

Codecov Report

Merging #6314 (e5560e7) into main (2c3f544) will not change coverage.
The diff coverage is n/a.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6314   +/-   ##
=======================================
  Coverage   94.22%   94.22%           
=======================================
  Files         111      111           
  Lines       23865    23865           
=======================================
  Hits        22487    22487           
  Misses       1378     1378