[PHP] Upgrade php-cs-fixer to 2.12, enables PHP >= 7.2 support

Created by: mattmelling

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Description of the PR

The PHP client uses friendsofphp/php-cs-fixer v1.12 for PSR2 analysis. 1,12 doesn't supports PHP up to < 7.2.

PHP >= 7.2 support was added in php-cs-fixer v2 which made some changes to the API and default fixers/rules.

This PR upgrades php-cs-fixer to v2.12. The fixers/rules have been aligned for PSR2 as desired in the original PR on swagger-codegen (#3863). I've changed defaults for some of the rules to prevent lots of whitespace warnings from php-cs-fixer, these typically seem to be down to blank template values and are not raised when using the configuration in master.

Notes

php-cs-fixer added the idea of "risky" fixers in v2. Since the strict_param and strict_comparison fixers are considered risky, we must alter the command line to vendor/bin/php-cs-fixer fix --dry-run --diff -vvv --allow-risky yes even when using --dry-run.

/cc @jebentier, @dkarlovi, @mandrean, @jfastnacht, @ackintosh, @ybelenko