Update compatibility matrix to include Go 1.21

[hariso]

Hey all!

Firstly, thanks to all the maintainers and contributors to this project! TinyGo is a great addition to the Go ecosystem.

Secondly, the compatibility matrix at https://tinygo.org/docs/reference/go-compat-matrix/ doesn't mention Go 1.21, but looking at the 0.29.0 release, Go 1.21 is supported.

[nikpivkin]

Matrix does not include tinygo versions 0.30.0 to 0.34.0