[RF] RooFit open issues and backports for 6.26.08 release

[guitargeek]

The usual issue to collect what is needed for RooFit in the next 6.26 patch release, this time for 6.26.08.

The related issues for the previous 6.26 releases:

1. https://github.com/root-project/root/issues/9508
2. https://github.com/root-project/root/issues/10112
3. https://github.com/root-project/root/issues/10493
4. https://github.com/root-project/root/issues/10758

GitHub issues that still need to be addressed for the next patch release:

• [ ] https://github.com/root-project/root/issues/10991
• [x] https://github.com/root-project/root/issues/11332
• [x] https://github.com/root-project/root/issues/11396

Items that still need to be done without any associated GitHub issue:

• [ ] use batchmode in stressRooFit also in createNLL (not only in fitTo) and fix bugs such that things don't crash
• [ ] make sure that this complicated conditional fit also gives the same result in BatchMode: https://root-forum.cern.ch/t/strange-behaviour-for-multidimensional-pdf-with-conditional-observables/51012/3
• [ ] Support the SplitRange command argument in the BatchMode. The unit test can be inspired by the reproducer code in #11396.
• [ ] Make sure no unit tests would fail if the BatchMode would be made the default (see #11400 for the current status of failing tests)

The bugfix PRs that need to be backported in this order to the 6.26 branch before the next 6.26 patch release:

• [x] https://github.com/root-project/root/pull/11023 Only the last commit, which has not been backported for the last patch release yet (the rest of PR was backported before the 6.26.04 release)
• [x] https://github.com/root-project/root/pull/11123 Only the first commit that relates to the batch mode
• [x] https://github.com/root-project/root/pull/11162
• [x] https://github.com/root-project/root/pull/11129 Only the first two commits that are note just code modernization
• [x] https://github.com/root-project/root/pull/11134
• [x] https://github.com/root-project/root/pull/11195
• [x] https://github.com/root-project/root/pull/11346
• [x] https://github.com/root-project/root/pull/11343 Only the first commit that is not code modernization
• [ ] https://github.com/root-project/root/pull/11401
• [ ] https://github.com/root-project/root/pull/11406 Only the last commit that changes the PDF cloning behavior, also excluding the testRooSimultaneous changes in that commit

The backports were grouped in a few PRs to v6-26-00-patches:

1. https://github.com/root-project/root/pull/11133
2. https://github.com/root-project/root/pull/11402