MSC4142: Remove unintentional intentional mentions in replies

[tulir]

Rendered

Implementations:

• https://github.com/matrix-org/matrix-react-sdk/pull/12511 (removed mention propagating from existing replied-to mention code)
• https://github.com/Nheko-Reborn/nheko/commit/cf373c016ad422d9a338209a45253455eb38a387 (added mention of replied-to user without mention propagating)

This MSC was inspired by an [unintentional mention chain in Matrix HQ](https://matrix.to/#/!OGEhHVWSdvArJzumhm:matrix.org/$XEN-hz_cQNCqXKbrPTPEVyykAGz-IYLstjSzzPFf33E?via=matrix.org&via=envs.net&via=maunium.net)


[tulir]

Discussion about the issue on the original MSC: https://github.com/matrix-org/matrix-spec-proposals/pull/3952#discussion_r1172839435