Streamline AppDirs paths

[koppor]

While discussing the backup path with @ThiloteE, I found, we are inconsistent.

Before:

├───100.0.0
│   └───backups
├───Logs
│   ├───100.0.0
│   ├───5.7--2022-08-05--73c111c
│   ├───5.8--2022-11-21--f7ea169
│   ├───5.8--2022-12-18--b7fae4b
│   ├───5.9--2023-01-04--bc3b9892f
│   ├───6.0--2023-01-01--b06ec3f
│   ├───6.0--2023-01-04--06771c8
│   └───unknown
├───lucene94
│   ├───-1446191503
│   ├───-1525049150
│   ├───-1981152844
│   ├───-2018577075
│   ├───-271657575
│   ├───-404722334
│   ├───-911001257
│   ├───1750725641
│   ├───1907816293
│   ├───647714185
│   ├───762261717
│   └───unsaved
└───ssl

After:

├───backups
├───logs
│   └───100.0.0
├───lucene94
│   ├───-1446191503
│   ├───-1981152844
│   ├───-2018577075
│   └───-404722334
└───ssl

Note that logs is spelled with lower-case l. This is consistent to ssl and lucene94. - We could discuss if we should introduce another level of hierarchy for lucene: lucene/94 to be consistent to logs.

Backups still work (checked by modifing a backup)

• Change in CHANGELOG.md described in a way that is understandable for the average user (if applicable)
• Tests created for changes (if applicable)
• Manually tested changed features in running JabRef (always required)
• Screenshots added in PR description (for UI changes)
• Checked developer's documentation: Is the information available and up to date? If not, I outlined it in this pull request.
• Checked documentation: Is the information available and up to date? If not, I created an issue at https://github.com/JabRef/user-documentation/issues or, even better, I submitted a pull request to the documentation repository.

[Siedlerchr]

Needs to be tested with snaps.
For lucene im against moving the path. Will trigger reindex

[koppor]

For lucene im against moving the path. Will trigger reindex

I can add backwards compatibility. In case the old path exists, use that. If there is none, use the newer one.

[koppor]

For lucene im against moving the path. Will trigger reindex

I can add backwards compatibility. In case the old path exists, use that. If there is none, use the newer one.

Or even more simpler: If lucene94 is used, use the path as is. If a newer version is used, use the new structure.

[koppor]

Note the current PR does not touch the lucene path. -- We could do that when we modify the index at #8963?

[koppor]

Other note: jabref is always written in lower case latter. Before, there was lowercase at start, but at other places mixed case.

[calixtus]

We are not honestly discussing backwards compatibility for temporary files? A refresher of the search index is not a bad thing. Of course, it takes some time to rebuild the index, if you install a new version of jabref every three to six months.