Go 1.21.0 was released in August 2023, so our upcoming release
will no longer support the Go 1.20 release series.
The first Go 1.22 release candidate is also due in December 2023,
less than a month from now, so dropping 1.20 will simplify 1.22 work.
The first makes our test scripts more consistent, as all external
program executions happen via "exec" and are not as easily confused
with custom builtin commands like our "generate-literals".
The second catches mistakes if any of our txtar files have duplicate
files, where all but one of the contents would be ignored before.
This way, the child process knows that it's running a toolchain command
via -toolexec without having to guess via filepath.IsAbs.
While here, improve the docs and tests a bit.
Following the best practices from upstream.
In particular, the "txt" extension is somewhat ambiguous.
This may cause some conflicts due to the git diff noise,
but hopefully we won't ever do this again.
Daniel Martí