CI: remove duplicit PHP version from test matrix #696

Job	Run time
Coding standard (ubuntu-latest, 8.4)	18s
Test for mutants (ubuntu-latest, 8.4)	2m 9s
Static analysis (ubuntu-latest, 8.4)	28s
Tests (ubuntu-latest, 7.4, false)	24s
Tests (ubuntu-latest, 8.0, false)	23s
Tests (ubuntu-latest, 8.1, false)	26s
Tests (ubuntu-latest, 8.2, false)	22s
Tests (ubuntu-latest, 8.3, false)	25s
Tests (ubuntu-latest, 8.4, false)	22s
Tests (ubuntu-latest, 7.4, --prefer-lowest --prefer-stable, false)	26s
Tests (macos-latest, 8.4, false)	1m 31s
Tests (windows-latest, 8.4, false)	1m 32s
Code coverage finish (ubuntu-latest, 8.4)	2s
Status check - CI	1s
	8m 49s

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