Github Actions: Drop testing for LLVM < v6.0.0

[Geod24]

Matches what LDC did recently.
Hopefully this will also reduce the cache load,
potentially resulting in less cache eviction,
and so less downloads.

CC @wilzbach @kinke

[dlang-bot]

Thanks for your pull request, @Geod24!

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Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + dmd#11537"

[Geod24]

We also should support LLVM 10, but I'll raise a separate PR for that.