[chore] Replace sigs.yml by workstreams.yml

[mx-psi]

This PR is a subset of #3337 in the hopes of finding one sub-part that can be merged.

It tries to move from sigs.yml to workstreams.yml without making any modifications on the TC sponsors or add non-SIG projects. The intent is to tackle all of these on separate PRs.

[jack-berg]

Looks good to me. I checked out locally, did a diff with my branch, and determined that this is the same as my branch, with the following changes:

• Strip out notion of working group from workstreams data model
• Strip out TC sponsorship assignments not already encoded in sigs.yml (Note, I didn't pull that out of thin air, the assignments came from this private TC sponsorship tracking doc. Still, makes sense to decouple.)
• Strip out make report-generation and resulting workstreams.md
• Other little things like keeping make validate-sigs instead of make validate-workstreams

Thanks for picking this up @mx-psi!

[mx-psi]

Looks good to me. I checked out locally, did a diff with my branch, and determined that this is the same as my branch, with the following changes:

Thanks for taking a look, sorry, I should have added a list like this to the README.

I intend to add all other changes you mention in future PRs, on this one I wanted to focus on changing the data model

[mx-psi]

I will merge this on Wednesday morning unless there are blocking comments before then

[jpkrohling]

LGTM. We could perhaps keep a sigs.yml pointing to the new one, but that's definitely not a blocker.

[mx-psi]

LGTM. We could perhaps keep a sigs.yml pointing to the new one, but that's definitely not a blocker.

I removed sigs.yml, sorry about that, I botched the merge. I think we can always revert back to sigs.yml if needed