ociregistry: drop internal/exp

Now that cuelang.org/go requires Go 1.21 or later,
we can also drop support for Go 1.20 in the oci repository.

The slices package can be replaced directly by std now.
The maps package is not as simple, as the Keys API was delayed
until iterators were stabilized, so for now, copy one function,
which is significantly easier and smaller too.

Signed-off-by: Daniel Martí <[email protected]>
Change-Id: Id7fc6d63c7b44f5ee5e86184d60b247c7c24852c
Reviewed-on: https://review.gerrithub.io/c/cue-labs/oci/+/1183479
TryBot-Result: CUE porcuepine <[email protected]>
Reviewed-by: Roger Peppe <[email protected]>

Author: mvdan

ociregistry/go.mod

@@ -1,6 +1,6 @@
 module cuelabs.dev/go/oci/ociregistry
 
-go 1.20
+go 1.21
 
 require (
 	github.com/go-quicktest/qt v1.100.0