Update test.yml for more recent Rubies

[bf4]

Add 3.1 and 3.2

[bf4]

I'm getting a compile error for rbtree on Ruby 3.1 and thought it would be worth checking if the sorted_set gem tests against Ruby 3.1 and to help figure out if it's me or not.

rbtree.c:763:25: error: cannot assign to non-static data member 'klass' with
const-qualified type 'const VALUE' (aka 'const unsigned long')
    RBASIC(temp)->klass = 0;
    ~~~~~~~~~~~~~~~~~~~ ^

.rbenv/versions/3.1.2/include/ruby-3.1.0/ruby/internal/core/rbasic.h:88:
17:
note: non-static data member 'klass' declared const here
    const VALUE klass;
    ~~~~~~~~~~~~^~~~~
rbtree.c:1418:9: error: call to undeclared function 'rb_inspecting_p'; ISO C99 and later
do not support implicit function declarations [-Wimplicit-function-declaration]
    if (rb_inspecting_p(self))
        ^
rbtree.c:1418:9: note: did you mean 'rb_inspect'?
.rbenv/versions/3.1.2/include/ruby-3.1.0/ruby/internal/intern/object.h:1
21:7:
note: 'rb_inspect' declared here
VALUE rb_inspect(VALUE obj);
      ^
rbtree.c:1420:12: error: call to undeclared function 'rb_protect_inspect'; ISO C99 and
later do not support implicit function declarations [-Wimplicit-function-declaration]
    return rb_protect_inspect(inspect_rbtree, self, str);
           ^
3 errors generated.
make: *** [rbtree.o] Error 1