refactor!(all the things): fuse `SyncTimelineEvent` into `TimelineEvent` #13692

Job	Run time
Generate Crypto FFI Apple XCFramework	5m 47s
Run Complement Crypto tests / tests	8m 6s
xtask / xtask-🐧	4s
xtask / xtask-🍏	10s
Test UniFFI bindings generation	1m 54s
matrix-rust-components-swift	11m 59s
matrix-rust-components-kotlin	4m 58s
	32m 58s

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