Spring Small fixes and improvements to the reference documentation

This PR fixes a few obvious typos and small issues in the reference documentation: - Remove mention that users could be on Java < 5 - Fix typo 'can can' - Fix subsection style in WebFlux Concurrency Model - Fix typo: Inn -> In - Fix links to Jakarta Mail 1.6 - Remove ref to JOTM, inactive since 2009

It makes 2 larger changes to improve cross-linking consistency between the MVC and WebFlux chapters: 1. Turn plain 'WebFlux' links to 'See equivalent in the Reactive stack' and turn plain 'WebMVC' links to 'See equivalent in the Servlet stack'

I found the links between chapters to be a little bit too terse, and there were some inconsistencies 2. Rework linking to Sprinv MVC Async support vs WebFlux section These links point to a section in the same chapter, but could be confused with the "WebFlux" links from above

The PR has been split into meaningful commits for ease of review, but these can be squashed upon merging.

Comment From: sbrannen

This work has been merged into main (with partial commit squashing, reordering, etc.).

Thanks