ref: replace str_starts_with with strpos in ClockMock

[Litarnus]

Replaces the usage of str_starts_with in our vendored ClockMock with strpos to remain PHP 7.2 compatible.

It currently only works because we are using polyfill through symfony/options-resolver, which will be removed in the future and will cause this code to fail.

[seer-by-sentry]

Potential bug: This change is an incomplete fix. Other calls to PHP 8+ functions like str_starts_with remain, which will cause fatal errors on older PHP versions.

• Description: The change at src/Util/ClockMock.php correctly replaces a PHP 8.0+ function call. However, this fix is incomplete. The codebase still contains calls to str_starts_with (in FrameBuilder.php) and str_contains (in DynamicSamplingContext.php). The PR author's belief that a polyfill is provided by symfony/options-resolver is incorrect. Since the project supports PHP 7.2+ and lacks a proper polyfill, these calls will cause fatal 'Call to undefined function' errors on PHP 7.2-7.4, crashing the application when processing stack traces or tracing headers.

• Suggested fix: Apply the same fix pattern to the remaining instances. Replace str_starts_with in FrameBuilder.php and str_contains in DynamicSamplingContext.php with PHP 7-compatible alternatives. Alternatively, add the symfony/polyfill-php80 dependency to provide these functions on older PHP versions.
  _{severity: 0.9, confidence: 0.95}

_{Did we get this right? 👍 / 👎 to inform future reviews.}