Change WG Comms references to WG Sustainability Advocacy and update old translations (#502)

[vallss]

This PR updates all the references of WG Comms to WG Sustainability Advocacy as part of the rebranding of the WG. In particular:

• change the folder name under working-groups and its references in the about/working-groups page
• change Slack channel name on every homepage, including translations

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[vallss]

@guidemetothemoon Not 100% sure whether the following errors will fix themselves after merging because we're actually changing the urls in this PR and obviously they now point to the not-yet-updated-links

ERROR: 3 dead links found! [✖] https://github.com/cncf/tag-env-sustainability/tree/main/working-groups/sustainability-advocacy → Status: [404](https://github.com/cncf/tag-env-sustainability/actions/runs/11767656936/job/32776584823?pr=576#step:9:406) [✖] https://github.com/cncf/tag-env-sustainability/tree/main/working-groups/sustainability-advocacy/charter.md → Status: 404 [✖] https://github.com/cncf/tag-env-sustainability/blob/main/working-groups/sustainability-advocacy/README.md#meetings-and-contact → Status: 404

[guidemetothemoon]

@vallss the check is running towards main branch so if links are updated as part of the PR the check may fail. But it will be OK once the PR is merged 😊

[guidemetothemoon]

LGTM, thanks!

[vallss]

@guidemetothemoon great, thanks! 💚