Make all workflows, reusable or otherwise, consolidate on .yml as a file extension

[thclark]

Feature request

Use Case

Sometimes, .yaml is creeping in as a file extension. Whilst common, it's not necessarily recognised as the official file extension (which should be .yml) and it's difficult to remember which are which.

Proposed solution

Rename all .yaml to .yml before this gets out of control, and establish convention of always using .yml to avoid ambiguity.