Spring Framework 7.0 requires a Servlet 6.1 baseline for Servlet containers.
Partial Servlet 6.1 support is available on the `main` Undertow branch,
but there isn't any milestone version available for the undertow 2.4
generation. At this stage of our 4.0 schedule, we think it's safer to drop
Undertow support now.
Closes: gh-46917
The enum itself is not specifically tied to auto-configuration so
spring-boot is a better place for it. The related condition,
`@ConditionalOnThreading`, remains in spring-boot-autoconfigure. This
aligns things with the similar `CloudPlatform` and
`@ConditionalOnCloudPlatform`.
Closes gh-46406
Phillip Webb
Brian Clozel
Stéphane Nicoll
Moritz Halbritter
Andy Wilkinson