Rename async index on ci_builds_metadata

What does this MR do and why?

Describe in detail what your merge request does and why.

This MR renames the index that was added to postgres_async_indexes in !108165 (diffs) to use the same name suggested in the follow-up MR. This should be OK since we're currently not processing async index requests until a problematic ci_builds index build is completed.

Part of #386755 (closed)

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