Update or remove `Last Change` dates

[laniakea64]

All our Vim syntax files have a Last Change date, which all currently claim nothing has changed since 2021 May 19. This is not true: there have been multiple major updates since then, even as far back as later in 2021 when this project was previously last active.

If we are going to have Last Change dates, we should plan to update them to reflect the actual dates of last change. Otherwise, we should remove them to prevent confusion stemming from inaccurate, obsolete metadata.

This could be important in the event that downstreams (e.g. vim-polyglot?) maybe using the Last Change date, where provided, to determine when to re-sync their packaging of vim-just?

If we are going to keep & update the Last Change dates:

To simplify the task of updating, I wrote a justfile recipe to update the Last Change dates based on git commit dates. This recipe should be added to this repo, but as it needs to be in a justfile located in the repo root, this is blocked by:

• https://github.com/NoahTheDuke/vim-just/pull/34

because that's where said justfile is going to be created.

Once the recipe for updating Last Change is in, then we can think about when to actually perform the update. Perhaps after the dust has settled from recent changes, so that the update of Last Change marks a point when we would suspect further immediate-future changes are relatively unlikely?

[laniakea64]

To simplify the task of updating, I wrote a justfile recipe to update the Last Change dates based on git commit dates. This recipe should be added to this repo,

Done in https://github.com/NoahTheDuke/vim-just/commit/07ad0d4695117a22bc13918ac62472bbfce0e1e5

[laniakea64]

keep & update the Last Change dates

Done in 9fc9a1afaa9e3567b25f4141a01f6172a1992a0b.