Raise Minimum Rustc Version to 1.61?

[mitsuhiko]

I believe that for filter/tests/global functions it would be nice to be able to borrow values. I finally found a way to make this work (#115) but unfortunately due to some bugs in rustc below 1.61 it does not seem possible to provide that API for older Rust versions.

Normally I like to be very conservative and support old rustc versions, but for this crate I at least personally do not have such a need.

So the question is if someone has the need MiniJinja to support old Rust versions or if bumping up to newer rusts is a possibility.

[mitsuhiko]

Since this crate has limited utilization beyond my own internal uses right now I think I will move up to 1.61.