stdlib: Update type for URI.join

URI.join takes not only strings, but also URI objects.  Therefore it
would be better to update its signature.

Internally, arguments will be converted to URI objects before
concatenating.
https://github.com/ruby/ruby/blob/v3_2_2/lib/uri/rfc3986_parser.rb#L107-L116

stdlib/uri/0/common.rbs

@@ -278,7 +278,7 @@ module URI
   #     URI.join('http://example.com', '/foo/', 'bar')
   #     # => #<URI::HTTP http://example.com/foo/bar>
   #
-  def self.join: (String str, *String strs) -> URI::Generic
+  def self.join: (_ToStr | URI::Generic str, *_ToStr | URI::Generic strs) -> URI::Generic
 
   # <!--
   #   rdoc-file=lib/uri/common.rb

test/stdlib/URI_test.rb

@@ -84,6 +84,8 @@ def test_join
                      URI, :join, "http://example.com"
     assert_send_type "(String, String) -> URI::Generic",
                      URI, :join, "http://example.com", "foo"
+    assert_send_type "(URI::Generic, URI::Generic) -> URI::Generic",
+                     URI, :join, URI("http://example.com"), URI("foo")
   end
 
   def test_parse