Deprecate 'graphql/subscriptions' and move code to 'execution'

[yaacovCR]

move the files from the subscription module into execution, in preparation for code integration

see:
#3195 (review)
#3195 (comment)

[IvanGoncharov]

@yaacovCR Thanks a lot, was a pleasure to review 👍
One thing, we use PR title in changelog so it should explain the effect of changes and not the intention of changes.
Even if in the future we will use another mechanism for maintaining changelog, I still think it will benefit users to have commit messages in that style.