Add aalto XML dependency to WebFlux starter

[bclozel]

As of SPR-14157, Spring Framework supports XML (de)serialization through blocking JAXB, or using aalto-xml which is fully non-blocking.

I'm wondering if we should add that dependency to the WebFlux starter given the following:

• aalto-xml weighs 350kb and its latest release is quite old now
• it's more or less the only available option for non-blocking XML parsing
• in the end, we rely on JAXB for the binding part
• it's not consistent with the jackson XML support we have in MVC right now, see Support non-blocking XML Marshalling with jackson-dataformat-xml #9581
• I have no idea if Jackson+Spring Framework will support non-blocking XML parsing and binding, like it's being done on the JSON side

[bclozel]

We're not going to include that by default in the WebFlux starter; we'll wait for spring-projects/spring-framework#19097. In the meantime, we should mention this dependency in the reference documentation.

[bclozel]

We're cleaning out the issue tracker and closing issues that we've not seen much demand to fix. Feel free to comment with additional justifications if you feel that this one should not have been closed.