fix: R8 compiler logs warnings due to missing ProGuard rules for OkHttp

[velkind]

New Pull Request Checklist

• [x] I am not disclosing a vulnerability.
• [x] I am creating this PR in reference to an issue.

Issue Description

Closes: https://github.com/parse-community/Parse-SDK-Android/issues/1221

Approach

Update OkHttp dependency from 4.10.0 to 4.12.0

TODOs before merging

• [x] No tests required
• [x] No documentation changes required

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[mtrezza]

Restarting the CI...