Support mtl-2.3

[Bodigrim]

$ cabal build -w ghc-9.6 --allow-newer -c 'mtl>=2.3'
Resolving dependencies...
Build profile: -w ghc-9.6.1 -O1
...
Building library for diagrams-html5-1.4.1..
[1 of 2] Compiling Diagrams.Backend.Html5

src/Diagrams/Backend/Html5.hs:87:48: error:
    Module ‘Control.Monad.State’ does not export ‘when’
   |
87 | import           Control.Monad.State          (when)
   |                                                ^^^^
Error: cabal: Failed to build diagrams-html5-1.4.1.

As a Hackage trustee I made a revision restricting mtl to <2.3 (and relaxing some other bounds):
https://hackage.haskell.org/package/diagrams-html5-1.4.1/revisions/

[byorgey]

Uploaded diagrams-html5-1.4.2 to Hackage.