Update dependency com.squareup.okhttp3:okhttp to v4.8.1

[renovate[bot]]

This PR contains the following updates:

Package	Update	Change
com.squareup.okhttp3:okhttp (source)	patch	4.8.0 -> 4.8.1

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Release Notes

square/okhttp

### [`v4.8.1`](https://togithub.com/square/okhttp/blob/master/CHANGELOG.md#Version-481) _2020-08-06_ - Fix: Don't crash in `HeldCertificate.Builder` when creating certificates on older versions of Android, including Android 6. We were using a feature of `SimpleDateFormat` that wasn't available in those versions!

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[mergify[bot]]

Nice! PR successfully merged.