Set minimum build target to 10.14

[jncn]

This allows for less version checks and further modernisation of the app.

Questions:

• Xcode recommends to enable some warnings. Given the age of the code base, I suspect it'll require quite some work to satisfy the compiler. Even though this will make the code base much stricter, I'd actually do this to prevent running in to trouble later.

Related issues:

• Replace use of deprecated WebView with WKWebView #37, since WebView is deprecated as from 10.14

[monkeydom]

I'm torn with the recommended warnings. I'm quite against nullability e.g. and also implicit conversion 32/64 and signed/unsigned usually just increases friction a lot and sometimes also adds bugs in the adoption as it needs to be carefully.

Overall: warnings need to be kept to 0 at all times, and the #37 already prevents this as of now. So it is not acceptable to enable warnings and not fix them at the same time to me, generally speaking.

[rtharston]

Xcode 14 added RECOMMENDED_MACOSX_DEPLOYMENT_TARGET environment variable and suggests using it in the build settings update prompt.

RECOMMENDED_MACOSX_DEPLOYMENT_TARGET is currently set to 10.14.6. Instead of updating to 10.14 how about using the new recommended target value of 10.14.6?

I'd understand if @monkeydom doesn't want to use RECOMMENDED_MACOSX_DEPLOYMENT_TARGET directly to avoid issues with it changing unexpectedly, but I think that if Apple has a reason to recommend 10.14.6 instead of 10.14 then there might be good reason for SubEthaEdit to jump to that version.