Update release schedule on README

[alamb]

Which issue does this PR close?

• related to Release object store 0.12.5 (maintenance) - Target Dec 2025 #582
• Related to Release object store 0.13.0 (breaking) - Target Nov 2025 #367

Rationale for this change

Update readme to reflect current plans

What changes are included in this PR?

Are there any user-facing changes?

[alamb]

Thank you @kylebarron

[tustvold]

Why do we direct users to delete_stream and copy_opts and not ObjectStoreExt?

[alamb]

This was meant for people who have implemented ObjectStore (not people who are calling)

What has happened is they now need to implement ObjectStore::delete_stream

The documentation on ObjectStore::delete_stream tries to explain what happened (and tries to explain how to port the code)

arrow-rs-object-store/src/lib.rs

Lines 955 to 960 in 44e258c

	/// Note: Before version 0.13, `delete_stream` has a default implementation
	/// that deletes each object with up to 10 concurrent requests. This default
	/// behavior has been removed, and each implementation must now provide its
	/// own `delete_stream` implementation explicitly. The following example
	/// shows how to implement `delete_stream` to get the previous default
	/// behavior.