Update to Twig 3.15.x

[MoritzLost]

Twig 3.15.0 brings a bunch of changes and features, most notably the ability to use arrow functions as callables for PHP functions that expect a function argument. This is very useful for collection methods, a lot of which require callables to be used.

Beyond that, there are a couple of deprecations and new syntax options. Upgrading the dependency will allow us to use the new syntax everywhere it's possible, and help us avoid using soon-to-be-deprecated language features.

Craft currently only allows ~3.14.

[AugustMiller]

Per Brandon's message in Discord—Twig 3.15 will ship with the upcoming 5.6 and 4.14 releases!