fix hour specs from deprecated "H" to "h"

[FabianHofmann]

Completely move the temporal specification from "H" to "h".

Checklist

• [x] Code changes are sufficiently documented; i.e. new functions contain docstrings and further explanations may be given in doc.
• [x] Unit tests for new features were added (if applicable).
• [x] Newly introduced dependencies are added to environment.yaml, environment_docs.yaml and setup.py (if applicable).
• [x] A note for the release notes doc/release_notes.rst of the upcoming release is included.
• [x] I consent to the release of this PR's code under the MIT license.