Update to go-1.21

[majst01]

This is long overdue, also removes the usage of 'x/exp'

The test error is caused by extended git security guards:

root@1b8f9afaebfe:/go/src/github.com/metal-stack/gardener-extension-provider-metal# git status
fatal: detected dubious ownership in repository at '/go/src/github.com/metal-stack/gardener-extension-provider-metal'
To add an exception for this directory, call:

        git config --global --add safe.directory /go/src/github.com/metal-stack/gardener-extension-provider-metal
root@1b8f9afaebfe:/go/src/github.com/metal-stack/gardener-extension-provider-metal#git config --global --add safe.directory /go/src/github.com/metal-stack/gardener-extension-provider-metal
root@1b8f9afaebfe:/go/src/github.com/metal-stack/gardener-extension-provider-metal# git status
On branch go-1.21
nothing to commit, working tree clean