Deprecate overriding the `__sleep` method of `ClassMetadata` or `Pers… #2556

Job	Run time
PHPUnit (8.3, 7.0, stable, server, highest, 7.4, lazy-ghost)	1m 16s
PHPUnit (8.4, 7.0, stable, server, highest, 7.4, lazy-ghost)	1m 13s
PHPUnit (8.2, 6.0, stable, server, highest, 7.4, lazy-ghost)	1m 10s
PHPUnit (8.5, 6.0, stable, server, highest, 7.4, lazy-ghost)	1m 21s
PHPUnit (replica_set, 8.2, 8.0, stable, highest, lazy-ghost)	1m 33s
PHPUnit (8.4, 8.0, stable, server, highest, 7.4, lazy-ghost)	1m 18s
PHPUnit (server, 8.1, 6.0, stable, highest, 6.4, lazy-ghost)	1m 18s
PHPUnit (8.5, 7.0, stable, server, highest, 7.4, lazy-ghost)	1m 20s
PHPUnit (8.3, 8.0, stable, server, highest, 7.4, lazy-ghost)	1m 23s
PHPUnit (lowest, 8.1, 6.0, 1.21.0, server, lazy-ghost)	2m 47s
PHPUnit (server, 8.4, 8.0, stable, highest, native, true)	1m 25s
PHPUnit (8.5, 8.0, stable, server, highest, 7.4, lazy-ghost)	1m 20s
PHPUnit (replica_set, 8.2, 6.0, stable, highest, lazy-ghost)	1m 24s
PHPUnit (8.4, 6.0, stable, server, highest, 7.4, lazy-ghost)	1m 15s
PHPUnit (8.2, 6.0, stable, highest, proxy-manager)	1m 27s
PHPUnit (8.4, 8.0, stable, highest, native)	1m 13s
PHPUnit (8.2, 8.0, stable, server, highest, 7.4, lazy-ghost)	1m 15s
PHPUnit (8.2, 7.0, stable, server, highest, 7.4, lazy-ghost)	1m 15s
PHPUnit (server, 8.2, 8.0, 1.21.0, highest, lazy-ghost)	2m 30s
PHPUnit (8.3, 6.0, stable, server, highest, 7.4, lazy-ghost)	1m 22s
PHPUnit (server, 8.4, 8.0, stable, highest, native)	1m 19s
	30m 24s

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