https://github.com/ruby/etc/commit/78987ce56a

Ruby 2.7 deprecates taint and it no longer has an effect.
The lack of taint support should not cause a problem in
previous Ruby versions.

Still untaint the tmpdir object on Ruby <2.7, as returning
a tainted string there could cause problems.

This removes the related tests, and puts the related specs behind
version guards.  This affects all code in lib, including some
libraries that may want to support older versions of Ruby.

This removes the security features added by $SAFE = 1, and warns for access
or modification of $SAFE from Ruby-level, as well as warning when calling
all public C functions related to $SAFE.

This modifies some internal functions that took a safe level argument
to no longer take the argument.

rb_require_safe now warns, rb_require_string has been added as a
version that takes a VALUE and does not warn.

One public C function that still takes a safe level argument and that
this doesn't warn for is rb_eval_cmd.  We may want to consider
adding an alternative method that does not take a safe level argument,
and warn for rb_eval_cmd.

We can check the function pointer passed to rb_define_module_function
like how we do so in rb_define_method.  The difference is that this
changeset reveales lots of atiry mismatches.

After 5e86b005c0f2ef30df2f9906c7e2f3abefe286a2, I now think ANYARGS is
dangerous and should be extinct.  This commit deletes ANYARGS from
rb_ensure, which also revealed many arity / type mismatches.