Add a deprecation warning when building with C++17

[jmcarcell]

BEGINRELEASENOTES

• Add a deprecation warning when building with C++17 before removing support for it in Require C++20 and update to C++20 #698

ENDRELEASENOTES

[andresailer]

Should we also set the default standard to 20 then, and not print the warning if the user doesn't do anything?

(cannot comment on the line directly)
https://github.com/AIDASoft/podio/pull/700/files#diff-1e7de1ae2d059d21e1dd75d5812d5a34b0222cef273b7c3a2af62eb747f9d20aR37

[tmadlener]

Should we also set the default standard to 20 then, and not print the warning if the user doesn't do anything?

Yes, definitely.

[jmcarcell]

Added, initially I thought about leaving all the changes to #698 (the warning only doesn't change anything compared to the default changing now).

[tmadlener]

The dev3 based workflows are failing for several PRs and I think is unrelated to the changes in any of those. From what I can tell from just looking at the outputs, it looks like we no longer recognize whether a file has a TTree or an RNTuple inside and it then instantiates the wrong reader in the interface:

https://github.com/AIDASoft/podio/actions/runs/11681697887/job/32527272599?pr=700#step:4:855

edit: they are most likely related to several changes to RNTuple for RC3.