Update bzlmod.md to remove unpinned_maven

[mbland]

This is no longer needed as of version 6.1, thanks to:

• Ensure @unpinned_maven//:pin continues to work with the maven resolver  #1112

Attn: @shs96c

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[shs96c]

@mbland It looks like you'll need to sign the CLA, either as yourself or for your employer.

[mbland]

@mbland It looks like you'll need to sign the CLA, either as yourself or for your employer.

I did: https://github.com/bazelbuild/rules_jvm_external/pull/1198/checks?check_run_id=27382993680

What happened was that I created the commit through the GitHub UI, which I suppose defaulted to my civilian account. I recreated it locally with my EngFlow address, force-pushed, and the CLA check passed.

[shs96c]

LGTM! Thank you!