Move testing-commons subproject to best-effort

[spiffxp]

After discussion with current testing-commons subproject owners
@alejandrox1 and @andrewsykim we have decided to move the
testing-commons subproject to best-effort.

It is important that we have oversight of kubernetes/kubernetes tests,
but folks now lack the bandwidth to consistently set and drive an agenda
in this area.

Best-effort means we are stopping the testing-commons meeting and
archiving the testing-commons slack channel, and ask those contributors
interested in this topic to bring discussion to the sig-testing meeting
and sig-testing slack channel.

While I was here, I went ahead and moved sig-testing related slack
config into its own directory.

/hold
for lazy consensus, deadline 3pm PT Friday (kubernetes-sig-testing@ thread)
/cc @BenTheElder @stevekuznetsov
for sig-testing approval
/cc @alejandrox1 @andrewsykim
for current subproject owner approval

FYI @kubernetes/steering

[spiffxp]

/cc @mrbobbytables @coderanger
for any opinions on the slack config changes

[spiffxp]

post-merge TODO:

• remove testing-commons meeting from zoom
• remove testing-commons meeting from sig-testing calendar

done

[spiffxp]

/hold cancel
lazy consensus deadline has passed

I've removed the testing-commons meeting from zoom and sig-testing's calendar

[BenTheElder]

/lgtm

[alejandrox1]

/lgtm

[spiffxp]

/assign @cblecker @nikhita @mrbobbytables @alisondy
for /approve

[nikhita]

/approve

/hold
for explicit review by slack admins. @mrbobbytables @coderanger can you take a look?

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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[mrbobbytables]

/lgtm

[spiffxp]

/hold cancel

[spiffxp]

Also closed the testing-commons project board: https://github.com/orgs/kubernetes/projects/44