Synapse is a bit confused when searching for messages which have been edited

[matrixbot]

This issue has been migrated from #8686.

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• Create an unencrypted room
• Send a message foo
• Search for foo (POST https://matrix-client.matrix.org/_matrix/client/r0/search) -> 1 result OK
• Edit the message from foo to bar
• Search for bar -> 1 result (bar) -> OK
• Search for foo -> 1 result (bar) -> KO

In Element Web:

Another scenario:

• Send a message foo test
• Search for test (POST https://matrix-client.matrix.org/_matrix/client/r0/search) -> 1 result OK
• Edit the message from foo test to bar test
• Search for test -> 2 results -> KO

I'm not sure about what is the proper fix, but I guess Synapse should only return the latest version of a message, and only if it contains the searched terms.