Docs: Update migration guide for v7 breaking changes

[martian7777]

What was wrong?

The migration guide was missing key details regarding breaking changes introduced in v7, specifically concerning the removal of the LegacyWebSocketProvider and the dropped support for Python 3.8 and 3.9.

Closes #3778

How was it fixed?

Updated docs/migration.rst to include:

• A new section noting that support for Python 3.8 and 3.9 has been dropped.
• Updated the WebSocketProvider section to explicitly state that the LegacyWebSocketProvider has been removed (previously marked as deprecated) and replaced by the V2 implementation.
• Clarified the migration path from WebsocketProviderV2 to WebSocketProvider.

Visuals

N/A (Documentation update only)

Checklist:

• Documentation added or updated
• Performed a self-review of my own code
• Built the documentation locally to ensure no warnings or errors

[kclowes]

This needs to go in a new v8 section. It won't be released in v7.

[martian7777]

@kclowes I will add a new section and will implement there!

[kclowes]

Great, thanks! I'm looking into those failures now, I don't think they have anything to do with your changes.

[martian7777]

@kclowes I updated with new docs section and i don't know why checks are failing.

[martian7777]

Additionally, I'm getting these warnings also when i run sphinx-build -b html . _build/html