Drop support for Jetty 9.4.21 and earlier

[philwebb]

See #18536

[dreis2211]

Do we have some documentation about minimum supported versions in general, @philwebb ? I recently noticed that Spring-Boot apps on Jetty 9.4.14.v20181114 don't work with Spring-Boot 2.2 anymore, for example. Not directly relevant for this issue, but the "earlier" might be less versions than one would think ;)

[wilkinsona]

We don't. Generally speaking it is only safe to assume that the version Boot uses by default and later maintenance versions are supported. This could be because Boot relies on a bug fix or because there's been a breaking change in a third-party maintenance release that causes a compatibility problem. You can read this issue as a little more than a reminder to remove the code that tolerates 9.4.21 and earlier.

[dreis2211]

That's what I thought as well. The issue reminded me of asking though.

[philwebb]

Yeah, this issue was mainly me reminding myself that in we'll be able to drop that reflection code in the Boot 2.3.

[snicoll]

Isn't that issue superseded somehow by #18842? I see that the related commit already removed the reflection code.

[wilkinsona]

We still have tests that verify compatibility with the older version.