Bring Spring benchmarks to latest generation.

[odrotbohm]

trafficstars

Updated the Spring benchmarks to the latest 3.2 release candidate. Removed obsolete manual configuration. Upgrade Java to 21 and switch back to Tomcat to benefit from virtual threads. Let Docker files consistently use Eclipse Temurin 21.

[sdeleuze]

I am working on additional refinements in collaboration with @odrotbohm, so please wait before merging this PR. We will comment when it will be reader to merge.

[NateBrady23]

@odrotbohm @sdeleuze Please use the draft functionality if it's not ready

[odrotbohm]

Switched to draft as recommended.

[odrotbohm]

@sdeleuze is taking over this one to further polish it and create a new request.