Make more transparent which spring addon repository is for what flow version

We had a triage session with the team, and we discussed it could be useful to have:
- an issue template 
- and some notes in README.md
about which Spring Addon is related to which flow and where it is stored. 
(Vaadin 23+ is in this repository: https://github.com/vaadin/flow/tree/main/vaadin-spring if I recall correctly from the meeing)