Improve build-type defaulting - #4958

A personal DRY-pet-peeve of mine when writing .cabal files from scratch is forgetting about declaring the build-type, which historically will then be inferred to be Custom which is almost always not what is desired. Instead, a better default is Simple as that represents the most basic and recommended build-type. Morever, starting with cabal-version:1.24, we have a redundant signal in the .cabal file to statically imply a custom build-type, namely the custom-setup stanza.

To this end, this PR finally removes this papercut by improving the defaulting to use the following simple rules:

• When cabal-version is set to 2.1 or higher, the default is Simple unless a custom-setup stanza exists, in which case the inferred default is Custom.
• For cabal-versions below 2.1, the default is Custom unconditionally (legacy defaulting).

This allows us to bring down the minimal (modern) trivial cabal package definition down to a single file with 4 lines:

cabal-version: 2.1
name: mu
version: 0
library

NB: We don't need any Setup.hs file, as cabal new-build doesn't need any, and cabal sdist will magically generate one on the fly and include it in the source distribution.

Please include the following checklist in your PR:

• Patches conform to the coding conventions.
• Any changes that could be relevant to users have been recorded in the changelog.
• The documentation has been updated, if necessary.
• Test-suite has been extended/updated