[EXPERIMENTAL] Drop C++11 and C++14 support

[rwgk]

Description

EXPERIMENTAL/WIP

(Not Draft only so that the full CI runs.)

Suggested changelog entry:

• Placeholder.

[henryiii]

A few notes:

• I want to make our next release first, then I'm personally fine with it, 17 is pretty widely supported now. I guess we'll have to find something for 11 so we can redefine what the 11 in our name means. :)
• The interesting thing would be to see how much this changes/drops/simplifies.
• This does raise our minimum macOS requirement on older CPythons (from memory, I think <3.12.4). That's been the reason I've avoided it so far. I think all other platforms are basically unaffected.

[rwgk]

A few notes:

• I want to make our next release first,

Sounds best to me, too.

• The interesting thing would be to see how much this changes/drops/simplifies.

I agree. But there is also the aspect that dropping C++11 and C++14 unshackles future developments. We're already at 9 years plus 17, it seems like a very long wait already. We could maintain 3.1.x (still C++11) for a couple more years, probably the effort for that will be less than what we'd pay for staying C++11 compatible on the mainline.

• This does raise our minimum macOS requirement on older CPythons (from memory, I think <3.12.4). That's been the reason I've avoided it so far. I think all other platforms are basically unaffected.

Ah, I didn't realize. That would be another argument for actively maintaining 3.1.x until Python 3.11 reaches EOL.