karma: stop listening on "::"

[pjonsson]

What this PR does

Karma-runner 6.3.20 and later defaults
to IPv4, so this removes the warning:

WARN [config]: ListenAddress was set to :: but hostname was left as the default: localhost. If your browsers fail to connect, consider changing the hostname option.

Test me

I think this is for @ljowen who added the workaround earlier, but just checking that the CI tests still work and the message disappears should be sufficient.

Checklist

• There are unit tests to verify my changes are correct or unit tests aren't applicable (if so, write quick reason why unit tests don't exist)
• I've updated relevant documentation in doc/.
• I've updated CHANGES.md with what I changed.
• I've provided instructions in the PR description on how to test this PR.