AthenaPoolCnvSvc: Switch back to ZLIB for tmp files
OK, I underestimated how ancient the ROOT version in release 21 is. It doesn't even have LZ4 since that is only introduced in v6-10-08 whereas we use v6-08-06. This should address ATLASRECTS-7504 after a full circle.
BTW, I should also note that we typically don't support forward compatibility. It's not only the technical ROOT things per se but a lot is different between master and release 21 as far as the core I/O is concerned, including but not limited to the DataHeader. I see Marcin introduced the new version of the DataHeader to release 21 in !24664 (merged) but I'm not sure if there are any other important fixes/changes (in DataHeader or elsewhere) that are not back-ported. I assumes things are thoroughly validated but we should be really careful not to be bitten by something more convoluted in general.