Go 1.22 is released with release notes!

It is time to shift the window for go support to [1.21, 1.22] by updating

• go. mod
• GitHub Actions
• README.md

After shifting the golang support window, we can remove / update some internal implementations using the latest golang built-in libraries introduced in golang 1.21.

• Remove the package oras.land/oras-go/internal/slices and replace it with the golang built-in package slices.
• Replace map clone logic in Map() and TagSet() of internal/resolver/memory.go with the built-in maps.Clone to improve performance.