Address warning: URI::RFC3986_PARSER warnings

This commit addresses the following warnings below.

```
/path/to/mail/lib/mail/spec/mail/attachments_list_spec.rb:179: warning: URI::RFC3986_PARSER.escape is obsoleted. Use URI::RFC2396_PARSER.escape explicitly.
/path/to/mail/lib/mail/spec/mail/utilities_spec.rb:386: warning: URI::RFC3986_PARSER.escape is obsoleted. Use URI::RFC2396_PARSER.escape explicitly.
/path/to/mail/lib/mail/spec/mail/utilities_spec.rb:390: warning: URI::RFC3986_PARSER.unescape is obsoleted. Use URI::RFC2396_PARSER.unescape explicitly.
/path/to/mail/lib/mail/utilities.rb:160: warning: URI::RFC3986_PARSER.escape is obsoleted. Use URI::RFC2396_PARSER.escape explicitly.
/path/to/mail/lib/mail/utilities.rb:164: warning: URI::RFC3986_PARSER.unescape is obsoleted. Use URI::RFC2396_PARSER.unescape explicitly.
/path/to/mail/lib/mail/utilities.rb:452: warning: URI::RFC3986_PARSER.unescape is obsoleted. Use URI::RFC2396_PARSER.unescape explicitly.
/path/to/mail/lib/mail/utilities.rb:463: warning: URI::RFC3986_PARSER.escape is obsoleted. Use URI::RFC2396_PARSER.escape explicitly.
```

Ruby 3.4 changes URI::DEFAULT_PARSER to URI::RFC3986_Parser and deprecates some methods.
URI::RFC3986_PARSER.make_regexp and URI::RFC3986_PARSER.make_regexp are used in the mail gem.

This commit uses URI::RFC2396_PARSER only if it is available for these versions:

- uri v0.12.2 for Ruby 3.2/3.1
- uri v0.13.1 for Ruby 3.3
- Ruby 3.4.0dev

https://bugs.ruby-lang.org/issues/19266

lib/mail/utilities.rb

@@ -165,7 +165,11 @@ def uri_unescape( str )
     end
 
     def uri_parser
-      @uri_parser ||= URI.const_defined?(:DEFAULT_PARSER) ? URI::DEFAULT_PARSER : URI
+      @uri_parser ||= if URI.const_defined?(:DEFAULT_PARSER)
+                        defined?(URI::RFC2396_PARSER) ? URI::RFC2396_PARSER : URI::DEFAULT_PARSER
+                      else
+                        URI
+                      end
     end
 
     # Matches two objects with their to_s values case insensitively
@@ -464,7 +468,7 @@ def Utilities.param_encode(str)
     end
 
     def Utilities.uri_parser
-      URI::DEFAULT_PARSER
+      defined?(URI::RFC2396_PARSER) ? URI::RFC2396_PARSER : URI::DEFAULT_PARSER
     end
 
     # Pick a Ruby encoding corresponding to the message charset. Most