[ci] Drop Emacs 26, test Emacs 30

[alexander-yakushev]

We have 1 person using Emacs 26 according to State of CIDER 2025 results. Stable debian (Bookworm) ships Emacs 28. Bullseye had 27.

• You've updated the changelog (if adding/changing user-visible functionality)
• You've updated the user manual (if adding/changing user-visible functionality)

[bbatsov]

I think it'd be better to just specify the indentation in the definition of the macro with:

(declare (indent 1))

I think this should solve the indentation issues here.

[alexander-yakushev]

The linter story looks harder than I thought. I'll probably revert to 28, too much work getting rid of all byte-compilation warnings.

[bbatsov]

Yeah, that's prudent. I guess I'll need to address those problems myself down the road.

[bbatsov]

That's fine by me, although you should probably update the Emacs dep in cider.el as well. Or we can just keep, until we actually need something from Emacs 27. (I don't recall it being an interesting release) Generally, nothing particularly useful for us happened until Emacs 29.

The indentation changes to cl-flet look somewhat weird to me, but I also saw them in another proposal, so I guess we'll have to stomach them. I'd like to avoid the awkward indentation in the selector methods, though. (at least to me it looks quite confusing)

[alexander-yakushev]

I reverted the linter back to 28, together with the indentation changes.

There is a 27 test failing, it also fails on master, so it could be something seasonal. Let's wait and see if it recovers later.

[alexander-yakushev]

@bbatsov I've commented Emacs 27 testing for now.

[bbatsov]

I think that's fine. I'm now wondering if the problem is not caused by something in Eldev's settings, but overall we can proceed as is for now.