Finalize version 2.6.0

This MR brings the final changes that make up for version 2.6.0, that will be released straight after merging.

Changes include:

• Adding frequency parameter to EvaluateConfig and FairnessConfig, enabling to skip some rounds.
• Improving user documentation as to recent features.
• Extending unit tests for FederatedServer, which is now >80% covered with unit tests only.
• Removing features that have been deprecated since version 2.4.0, as planned.
• Adding release notes, incrementing the version number, etc.

On my local computer, that has no GPU (but not using --cpu-only), tests achieve:

• 92.87 % coverage running unit tests only, with torch2
• 93.24 % coverage adding re-run tests with torch1
• 95.43 % coverage adding --fulltest functional tests