As a semantics, Hash#each yields a 2-element array (pairs of keys and
values).  So, `{ a: 1 }.each(&->(k, v) { })` should raise an exception
due to lambda's arity check.
However, the optimization that avoids Array allocation by using
rb_yield_values for blocks whose arity is more than 1 (introduced at
b9d29603375d17c3d1d609d9662f50beaec61fa1 and some commits), seemed to
overlook the lambda case, and wrongly allowed the code above to work.

This change experimentally attempts to make it strict; now the code
above raises an ArgumentError.  This is an incompatible change; if the
compatibility issue is bigger than our expectation, it may be reverted
(until Ruby 3.0 release).

[Bug #12706]

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.