[php-slim4] Switch to Packaged Body Parsing Middleware

[ybelenko]

Remove custom JSON parsing middleware and enable builtin recommended solution.
Ref: Body Parsing Middleware - Slim Framework

Closes #9337

Tests locally:

composer test && composer phpcs
> phpunit
PHPUnit 9.5.4 by Sebastian Bergmann and contributors.

Warning:       Your XML configuration validates against a deprecated schema.
Suggestion:    Migrate your XML configuration using "--migrate-configuration"!

IIIIIIIIIIIIIIIIIIIIIIII.III.III.III.IIIIIII.IIIIIII.III.IIIIII  63 / 120 ( 52%)
III......................................................       120 / 120 (100%)

Time: 00:00.085, Memory: 8.00 MB

OK, but incomplete, skipped, or risky tests!
Tests: 120, Assertions: 143, Incomplete: 59.
> phpcs
............................... 31 / 31 (100%)


Time: 2.13 secs; Memory: 12MB

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cc @jebentier, @dkarlovi, @mandrean, @jfastnacht, @ackintosh, @renepardon
cc Slim4 team member @l0gicgate

[ybelenko]

Just rebased on the latest master in hope to fix CI errors, no changes from original PR.

UPD: rebased again at November 23, 2021 to fix CI errors.
UPD: rebased again at December 5, 2021 to fix CI errors.

[wing328]

Sorry for the delay in merging the PR. 🙇‍♂️