Bump api-tools to get fix for changing base types. (#19356)

New commits in rolfbjarne/api-tools:

* rolfbjarne/api-tools@6d4007a [mono-api-html] Allow changing the base type if the old base type is a super class of the new base type.
* rolfbjarne/api-tools@cbc1e08 [mono-api-html] Fix compiler warning about obsolete formatter-based serialization attributes.

Diff: c7d5208967..6d4007a599

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Co-authored-by: Alex Soto <alex@alexsoto.me>

external/api-tools

@@ -1 +1 @@
-Subproject commit c7d5208967d6d46fdebcac82ade488f00e1fcc50
+Subproject commit 6d4007a5990ef324ca9f121ce6d0260efc7dcf94