fix: php deprecated log error level

[ishiguro-junya]

Issue #, if available: #2952

Description of changes:
Fix the log level.

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[stobrien89]

Hi @ishiguro-junya,

Thanks for the contribution! The decision to use E_USER_WARNING over E_USER_DEPRECATED was actually due to the fact that customers using versions in the affected deprecation ranges will still be able to continue using their current SDK versions; they just won't receive updates after we change the required PHP language level. We're going to keep the warning level at E_USER_WARNING, since this is not a deprecation in the sense of "code stops working".

[stobrien89]

Disregard my first comment. It was posted before I read the discussion in #2952. I'm fine with changing this.

[stobrien89]

Thanks again @ishiguro-junya!