Deprecate properties based on our resolution for user reading preferences

[HadrienGardeur]

Section

https://w3c.github.io/epub-specs/epub34/authoring/#sec-fxl-package

Describe the problem

During the 2025 TPAC in Kobe, we voted on the following resolution:

RESOLUTION: EPUB should prioritize user reading preferences for content appearance and reading experience

I previously opened a number of related issues earlier this year:

• Deprecating controls over orientation #2751
• Deprecating flow as a metadata #2754
• Deprecating flow overrides #2750
• Deprecating controls over the use of spreads #2753

@iherman also started working on a PR post-TPAC: #2831

Describe the fix or new feature you propose

Based on this resolution, the following properties should be deprecated in our next revision:

• rendition:orientation and the equivalent values for spine overrides
• rendition:flow and the equivalent values for spine overrides
• rendition:spread and the equivalent values for spine overrides