Add --ignore-vcs-tag for old version support purpose

[unkcpz]

we have a use case that after the new version 23.05.0 is released, we using branch support-23.04.x for cherry-pick commits and making releases to the old version.
However, running bumpver update --set-version v23.04.2 to bump the version from old version, errors raise because of VCS version in the remote is higher:

╰─± bumpver update --set-version v23.04.4 --dry                     
INFO    - fetching tags from remote (to turn off use: -n / --no-fetch)
INFO    - Working dir version        : v23.04.2
INFO    - Latest version from VCS tag: v23.10.0a0
ERROR   - Invariant violated: New version must be greater than old version 
ERROR   -   Failed Invariant: 'v23.04.4' > 'v23.10.0a0'
ERROR   - Invalid argument --set-version='v23.04.4'

I add --ignore-version-conflicts option to CLI to skip this check, test for my package and all works as expected.

[mbarkhau]

If you have the time, there is some housekeeping still left:

• Is there a better/shorter name you can think of?
• Can you write a test case? I think you can base it on test/test_cli.py#L930.
• Run scripts/update_readme_examples.py.
• Update the CHANGELOG.md

[unkcpz]

Okay, done!
I make the ignore slightly more specific. The option is --ignore-vcs-tag which only ignores the tags fetched from the remote but still keeps validating w.r.t cfg.current_version. The test is composed by mock the get_latest_vcs_version_tag.
Need to mention that --no-fetch not working in this case since I have a new version tag locally released from another branch.

[mbarkhau]

LGTM, thank you 👍