Up minimum compat for v1 to 1.10 (current LTS), test "min" and "lts" on CI

[Ickaser]

Description

On the current master, Plots has a Julia compat of 1.6.
However, extensions have been used in Plots for a while; it seems like the burden of loading them on old Julia hasn't been too high. But for #5174, the lack of extensions for dependencies of extensions starts to make maintenance of new code pretty difficult. A possible compat bound for getting the extensions working would be 1.9, but since 1.10 has been the LTS for almost a year now, I propose that gets used as a lower bound for testing and compat on new versions of Plots.

This PR updates that in Project.toml and the CI pipeline (where "lts" and "1" are now the only versions tested on all platforms). I also drop the block in Plots.__init()__ where extensions were loaded with Requires for old versions of Julia.

Corresponding changes have already been made on the v2 branch, it looks like, so this PR doesn't need a companion.

Attribution

• I am listed in .zenodo.json (see How to get acknowledged #3503)

[Ickaser]

@t-bltg I imagine you will want to weigh in on this

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.57%. Comparing base (fa65e7d) to head (ba4be8e).
⚠️ Report is 79 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5184      +/-   ##
==========================================
- Coverage   89.77%   88.57%   -1.20%     
==========================================
  Files          40       40              
  Lines        8780     8884     +104     
==========================================
- Hits         7882     7869      -13     
- Misses        898     1015     +117     

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[t-bltg]

@t-bltg I imagine you will want to weigh in on this

I think too that it's reasonable to ask to for julia 1.10 lts as a minimal version for a future Plots version of 1.41, but I'll let @BeastyBlacksmith make the final choice.

[BeastyBlacksmith]

Can we get rid of Requires.jl then, or is it used for something else still?

[Ickaser]

The other remaining usage of Requires is for some backend loading behavior. I didn't investigate that too closely, especially after seeing that v2 does away with that altogether (and already has a minimum julia 1.10 compat) in favor of extensions for backends. I could maybe take another look and see what exactly it's doing

[Ickaser]

This macro is used elsewhere in the file for backend loading:

Plots.jl/src/backends.jl

Lines 96 to 103 in 8d8965d

	macro require_backend(pkg)
	be = QuoteNode(Symbol(lowercase("$pkg")))
	return quote
	backend_name() === $be || @require $pkg = $(_plots_deps["$pkg"]) begin
	include(_path($be))
	end
	end |> esc
	end

[t-bltg]

Yeah in v2 backend code loading is done through extensions and this might no be easy to backport v2changes into v1.

[BeastyBlacksmith]

Alright, thanks for checking