Support get requests for /actuator/startup

[philwebb]

Currently the startup actuator endpoint only supports POST because it clears the buffer. We should offer a GET version that leaves the buffer in place. This will be easier to call call because CSRF protection won't be needed.

[nfrankel]

Subscribed 😅

[jenshadlich]

I understand that POST makes sense because of the side effects. But it's totally unintuitive because it's about "retrieving" (like info or threaddump). So yes, please support GET.

[bclozel]

Superseded by #24717