CompatHelper: bump compat for BlockBandedMatrices to 0.12, (keep existing compat)

[github-actions[bot]]

This pull request changes the compat entry for the BlockBandedMatrices package from 0.11 to 0.11, 0.12. This keeps the compat entries for earlier versions.

Note: I have not tested your package with this new compat entry. It is your responsibility to make sure that your package tests pass before you merge this pull request.

[codecov[bot]]

Codecov Report

Patch coverage has no change and project coverage change: -0.20 :warning:

Comparison is base (1cba88a) 67.98% compared to head (fb03eb3) 67.79%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #66 +/- ## ========================================== - Coverage 67.98% 67.79% -0.20% ========================================== Files 24 24 Lines 1690 1621 -69 ========================================== - Hits 1149 1099 -50 + Misses 541 522 -19 ``` [see 14 files with indirect coverage changes](https://codecov.io/gh/JuliaApproximation/CompactBases.jl/pull/66/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaApproximation) Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaApproximation). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaApproximation)

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