Update cimg/php Docker tag to v8.1

[renovate]

This PR contains the following updates:

Package	Type	Update	Change
cimg/php	docker	minor	8.0 -> 8.1
cimg/php	docker	major	7.4 -> 8.1
cimg/php	docker	major	7.3 -> 8.1
cimg/php	docker	major	7.2 -> 8.1
cimg/php	docker	major	7.1 -> 8.1

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