Fix keyword splat passing as regular argumentHEADmaster

Since Ruby 3.0, Ruby has passed a keyword splat as a regular
argument in the case of a call to a Ruby method where the
method does not accept keyword arguments, if the method
call does not contain an argument splat:

```ruby
def self.f(obj) obj end
def self.fs(*obj) obj[0] end
h = {a: 1}
f(**h).equal?(h)  # Before: true; After: false
fs(**h).equal?(h) # Before: true; After: false

a = []
f(*a, **h).equal?(h)  # Before and After: false
fs(*a, **h).equal?(h) # Before and After: false
```

The fact that the behavior differs when passing an empty
argument splat makes it obvious that something is not
working the way it is intended.  Ruby 2 always copied
the keyword splat hash, and that is the expected behavior
in Ruby 3.

This bug is because of a missed check in setup_parameters_complex.
If the keyword splat passed is not mutable, then it points to
an existing object and not a new object, and therefore it must
be copied.

Now, there are 3 specs for the broken behavior of directly
using the keyword splatted hash.  Fix two specs and add a
new version guard. Do not keep the specs for the broken
behavior for earlier Ruby versions, in case this fix is
backported. For the ruby2_keywords spec, just remove the
related line, since that line is unrelated to what the
spec is testing.

Co-authored-by: Nobuyoshi Nakada <nobu@ruby-lang.org>

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