[ntuple] remove checksum field from anchor

[silverweed]

Checksum is now excluded from the on-disk representation and it's not stored in the anchor. This allows to leverage the ROOT I/O machinery for schema evolution rather than having to rely entirely on custom code in RNTuple::Streamer().

The RNTuple class version gets bumped from 4 to 5.

This is a breaking change, but backwards compatibility with version 4 will be preserved for a while for easing the transition. Support for version 4 will be dropped before stabilization.

Checklist:

• [x] tested changes locally
• [ ] updated the docs (if necessary)

[github-actions[bot]]

Test Results

     9 files       9 suites   2d 4h 3m 21s :stopwatch:  2 636 tests  2 634 :white_check_mark: 0 :zzz: 2 :x: 22 602 runs  22 593 :white_check_mark: 0 :zzz: 9 :x:

For more details on these failures, see this check.

Results for commit ddc93d0c.

:recycle: This comment has been updated with latest results.