[Serializer] Use default context EXCLUDE_FROM_CACHE_KEY for unsetting OBJECT_TO_POPULATE

[alanpoulain]

Q	A
Branch?	4.4
Bug fix?	yes
New feature?	no
Deprecations?	no
Tickets	N/A
License	MIT
Doc PR	N/A

Following #36332, instead of unsetting directly the OBJECT_TO_POPULATE key from the context in getCacheKey, we can add the key to the EXCLUDE_FROM_CACHE_KEY array of the default context for Symfony >= 4.4.

[nicolas-grekas]

I think we should reject this PR:

1. it has a BC breaking potential (merging it for ppl that set the EXCLUDE_FROM_CACHE_KEY key will not unset(OBJECT_TO_POPULATE) anymore).
2. nobody complained that it could not use the OBJECT_TO_POPULATE as part of the cache key (I fail to see what the use case would be).

[dunglas]

I agree with @nicolas-grekas, let's close for now. We can still do it in a new PR if it becomes necessary at some point. Thanks for contributing @alanpoulain!