Merge pull request juju#11651 from wallyworld/increment-to-2.8.0

version bump to 2.8.0

Merge pull request juju#11556 from hpidcock/fix-gomod-tests

juju#11556

## Update juju/testing dep and fix test with compare

Updates juju/testing with better HOME detection with tests that abuse env vars.
Fix cloudImageMetadataSuite.TestSupportedArchitectures array compare.

## QA steps

Run unit tests with juju-db snap installed.

## Documentation changes

N/A

## Bug reference

N/A