Fix `EventLoopFuture` and `EventLoopPromise` under strict concurrency checking #642

Job	Run time
Soundness / Broken symlinks check	4s
Soundness / API breakage check	2m 25s
Soundness / Documentation check	2m 4s
Soundness / Format check	51s
Soundness / License headers check	8s
Soundness / Unacceptable language check	3s
Soundness / Shell check	38s
Soundness / Python lint check	11s
Soundness / YAML lint check	4s
Swift 6 Language Mode / Swift 6 language mode	42s
Integration Tests / Linux (5.9)	15m 17s
Integration Tests / Linux (5.10)	13m 54s
Integration Tests / Linux (6.0)	15m 31s
Integration Tests / Linux (nightly-6.0)	16m 22s
Integration Tests / Linux (nightly-main)	17m 5s
Unit tests / Unit tests / Linux (5.9)	3h 5m 28s
Unit tests / Unit tests / Linux (5.10)	3h 5m 23s
Unit tests / Unit tests / Linux (6.0)	3h 5m 22s
Unit tests / Unit tests / Linux (nightly-6.0)	3h 5m 26s
Unit tests / Unit tests / Linux (nightly-main)	3h 5m 21s
Benchmarks / Benchmarks / Linux (5.9)	5m 31s
Benchmarks / Benchmarks / Linux (5.10)	5m 3s
Cxx interop / Cxx interop / Linux (5.9)	1m 13s
Benchmarks / Benchmarks / Linux (6.0)	5m 28s
Cxx interop / Cxx interop / Linux (5.10)	1m 10s
Cxx interop / Cxx interop / Linux (6.0)	1m 36s
Benchmarks / Benchmarks / Linux (nightly-6.0)	6m 39s
Cxx interop / Cxx interop / Linux (nightly-6.0)	1m 39s
Benchmarks / Benchmarks / Linux (nightly-main)	6m 26s
Cxx interop / Cxx interop / Linux (nightly-main)	1m 56s
	17h 29m 0s

Provide feedback