[FOLLOWUP] Use the PHP 7.2 language level for Rector (#776) #1561

Job	Run time
PHP Lint (7.2)	12s
PHP Lint (7.3)	17s
PHP Lint (7.4)	18s
PHP Lint (8.0)	18s
PHP Lint (8.1)	10s
PHP Lint (8.2)	23s
PHP Lint (8.3)	22s
PHP Lint (8.4)	16s
Static Analysis (composer:normalize, 8.3)	17s
Unit tests (7.2)	10s
Static Analysis (php:fixer, 8.3)	18s
Unit tests (7.3)	10s
Static Analysis (php:stan, 8.3)	22s
Unit tests (7.4)	10s
Static Analysis (php:rector, 8.3)	22s
Unit tests (8.0)	13s
Unit tests (8.1)	11s
Unit tests (8.2)	11s
Unit tests (8.3)	10s
Unit tests (8.4)	21s
	5m 11s

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