dev docs list environment variabls as introduced in version 'next'.

[jnweiger]

Describe the bug

https://docs.opencloud.eu/docs/dev/server -> Services has a section 'Environment Variables' in each of the listed services.

These services have one or more environment variables that say 'next' for their introduction version:

• ActivityLog
• Collaboration
• Graph
• Postprocessing
• Search
• Storage-users
• Users
• Web

Expected behavior

Pleas review all occurrences of next, and substitute with a concrete version number.

Actual behavior

It is not clear, if next means the current 4.0.0 release or something later or earlier.

[micbar]

Correct: that depends on #1926

[micbar]

Then we need https://github.com/opencloud-eu/markdown-docs-generator/pull/6

[micbar]

And a new run of the docs generator.

@Svanvith @schweigisito