Fix mac build

[koppor]

Follow-up to JabRef#10338.

We either need this fix - or add an explanation in the README.md how to "properly" check out the code to have IntelliJ working (one commit in JabRef#10322 does this). Since I more want to have the guideline "intuitive", I would like to have this change in.

The installer file names and the startup scripts will be all-lowercase now. Which I kind of like. (bin/jabref instead of bin/JabRef)

Update casing of "JabRef"

• Lowercase "jabref" in scripts and binary names
• change vendor to JabRef e.V.
• Case "mac-package-identifier" to "JabRef"
• Group macos steps together

Mandatory checks

• 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.