all: add Go 1.21, drop Go 1.19
Per our Go support policy accepted in https://cuelang.org/issue/1937,
the upcoming CUE v0.7 release will require Go 1.20 or later.
Note that the files generated by gocode contain slightly smaller data,
which is due to a minor non-breaking change in encoding/gob in Go 1.21.
For more information, see https://cuelang.org/cl/1167483.
Signed-off-by: Daniel Martí <mvdan@mvdan.cc>
Change-Id: I63dc3d443feedf899b632320cce8060b925accec
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1167484
Reviewed-by: Paul Jolly <paul@myitcv.io>
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>
Unity-Result: CUE porcuepine <cue.porcuepine@gmail.com>