Go1.21 already has the fifth minor revision (released 2023-12-05) and
should therefore already be sufficiently stable. Furthermore we need it
to fix a bug in a NEO/go dependency [1].

Please find all details in the official release note:

    https://go.dev/doc/go1.21

It was released on 2023-08-08 [2]. Due to the go promise of compatibility
most software should still compile without any problems.

In golang < 1.21 we needed to patch golang to fix
https://github.com/golang/go/issues/42525. In golang 1.21 we still need
to apply a fix, but can't apply the old patch because the code changed.
In golang > 1.21 this problem is already fixed with https://go-review.googlesource.com/c/go/+/520055.

Because of https://github.com/golang/go/commit/092671423cd95eaa6df93eb29442fef41504d097
'TestUnshareMountNameSpace' fails on golang 1.21 [3]. In golang 1.20 this test
was skipped [4]. To fix this failure the additional patch
'skip-unshare-mount-test.patch' has been added.

---

[1] wendelin.core!22 (comment 195769)]
[2] https://go.dev/doc/devel/release
[3] --- FAIL: TestUnshareMountNameSpace (0.18s)
    exec_linux_test.go:243: unshare failed: exit status 2
        unshare: mount /tmp/TestUnshareMountNameSpace2210137852/001 failed: 0x1
[4] === RUN TestUnshareMountNameSpace exec_linux_test.go:333: kernel prohibits unshare in unprivileged process, unless using user namespace — SKIP: TestUnshareMountNameSpace (0.00s)

/reviewed-by @kirr and @jerome
/reviewed-on !1494