switch to golang standard library for slices

[dims]

slices package has been available since golang 1.21 [1]. current supported versions of golang is 1.22 and 1.23 [2].

So, Can we please bump

• at least 1.21 in go.mod
• switch the import to drop the experimental x/exp package

Context: in kubernetes, we have been trying to remove the usage of x/exp for a while now [3] and this is the last bit we need. thanks!

cc @liggitt

[1] https://tip.golang.org/doc/go1.21#slices
[2] https://endoflife.date/go
[3] kubernetes/kubernetes#128571