Remove experimental status from several features

Description

Several features are not experimental any more:

• DFT+U
• Casida triplets
• Parsing of XML pseudopotentials
• Time-propagation with k-point symmetries

News snippet

Remove experimental status from several features.

Checklist

• I have checked that my code follows the Octopus coding standards
• I have added tests for all the new features added in this request.

changed milestone to %11.1

assigned to @nicolastd

approved this merge request

enabled an automatic merge when the pipeline for 34c054d7 succeeds

merged

mentioned in commit 5a1d68a5