RelNotes: update the v0 with extension situation

With the two-patch series for regression fix, to the users from 2.27
days, there is no visible behaviour change---we do not warn and fail
use of v0 repositories with newer extensions yet, so there is nothing
to note in the backward compatibility section.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/RelNotes/2.28.0.txt

@@ -9,15 +9,6 @@ Backward compatibility notes
  * "fetch.writeCommitGraph" is deemed to be still a bit too risky and
    is no longer part of the "feature.experimental" set.
 
- * It used to be that setting extensions.* configuration variables
-   alone, while leaving core.repositoryFormatVersion=0, made these
-   settings effective, which was a wrong thing to do.  In version 0,
-   there was no special meaning in extensions.* configuration
-   variables.  This has been corrected.  If you need these repository
-   extensions to be effective, the core.repositoryFormatVersion
-   variable needs to be updated to 1 after vetting these extensions.*
-   variables are set correctly.
-
 
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