Support Map payloads in FormHttpMessageConverter

Prior to this commit, the `FormHttpMessageConverter` would only support
`MultiValueMap` payloads for reading and writing. While this can be
useful when web forms contain multiple values under the same key, this
prevent developers from using a common `Map` type and factory methods
like `Map.of(...)`.

This commit relaxes constraints in `FormHttpMessageConverter` and
ensures that `Map` types are supported for reading and writing URL
encoded forms.
Note that when reading forms to a `Map`, only the first value for each
key will be considered and other values will be dropped if they exist.

Closes gh-36408

framework-docs/modules/ROOT/pages/web/webmvc/message-converters.adoc

@@ -26,6 +26,7 @@ By default, this converter supports all text media types(`text/{asterisk}`) and
 | An `HttpMessageConverter` implementation that can read and write URL encoded forms.
 By default, this converter reads and writes the `application/x-www-form-urlencoded` media type.
 Form data is read from and written into a `MultiValueMap<String, String>`.
+`Map<String, String>` is also supported, but multiple values under the same key will be ignored.
 
 | `MultipartHttpMessageConverter`
 | An `HttpMessageConverter` implementation that can read and write multipart messages.