Bump kotlin version in multiple_flutters benchmark test

[camsim99]

Bumps the Kotlin version of the multiple_flutter benchmark test to fix #109397 when trying to bump the path_provider Gradle version.

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[stuartmorgan-g]

LGTM.

Are we able to successfully roll the path_provider_android pin forward with this change?

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[stuartmorgan-g]

Oops, I thought I had approved this!

[camsim99]

Are we able to successfully roll the path_provider_android pin forward with this change?

@stuartmorgan No worries! I tested out multiple_flutters with this change and the Gradle version change in path_provider locally, and it should be good to go. I'm going to follow this PR with bumping the rest of the integration test's Gradle and Kotlin versions where applicable to avoid this sort of issue in the future.