Set MSRV in manifest to actual current value

1.63.0 is the MSVR of the libm crate version used in our manifest and
seems to be the current lowest common denominator. It is only used when
building no-std, but I don't see any documentation that the MSVR for
this project can/should be separate for default features vs. non-default
features.

Author: alerque

Cargo.toml

@@ -10,6 +10,7 @@ repository = "https://github.com/RazrFalcon/ttf-parser"
 documentation = "https://docs.rs/ttf-parser/"
 readme = "README.md"
 edition = "2018"
+rust-version = "1.63.0"
 exclude = ["benches/**"]
 
 [dependencies]