Move "What's New" reference manual chapter to the wiki [SPR-14449]

[spring-projects-issues]

Sam Brannen opened SPR-14449 and commented

1. Create a "What's New" wiki page.
   • https://github.com/spring-projects/spring-framework/wiki/What's-New-in-the-Spring-Framework
2. Link to the existing "What's New" content for 3.x and 4.x from the new wiki page.
3. Link to the new wiki page from the appendix of the reference manual.
4. Delete the existing "What's New" chapter from the reference manual.
5. Begin documenting "What's New in Spring Framework 5.x" in the wiki.

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Affects: 5.0 M1

Referenced from: commits c341b39