Check if opcache_reset function exists

[felixarntz]

Bug Description

The opcache_reset() function may not be present on all systems which can result in fatal errors (see https://wordpress.org/support/topic/undefined-function-opcache_reset-v1-3-0/). This is a follow-up to #1066.

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Acceptance criteria

• There should be a function_exists check before calling opcache_reset.

Implementation Brief

• There should be a function_exists check before calling opcache_reset in googlesitekit_opcache_reset.

QA Brief

• N/A

Changelog entry

• Ensure opcache_reset() exists before calling it.

[aaemnnosttv]

IB ✅