Add triage-incoming.yml

[Half-Shot]

Taken from https://github.com/matrix-org/synapse/blob/develop/.github/workflows/triage-incoming.yml

Example usage in https://github.com/matrix-org/matrix-appservice-bridge/actions/runs/3274938737

[DMRobertson]

There are some warnings in the linked run---don't know if they're something we should worry about? Otherwise LGTM.

Would you mind changing Synapse to point to this?

Also, are you up for doing a release? (Merge PR, create github release with semver bump, force push the v1 tag to point at the release's tag)

[Half-Shot]

The warning looks like a fault with the module (it seems to do the same thing with Synapse, and we're definitely using the intended syntax https://github.com/matrix-org/synapse/actions/runs/3274133082)

Would you mind changing Synapse to point to this?

Yeah, can do post release.

Also, are you up for doing a release? (Merge PR, create github release with semver bump, force push the v1 tag to point at the release's tag)

Yeah, can do!

[DMRobertson]

Thanks!