Bump protobuf to 3.15.1 to work around Bazel 4 incompatibility

[johnboiles]

I ran into this issue with building MediaPipe using Bazel 4.0.0. Apparently you need to use at least Protobuf 3.13 with Bazel 4.0.0.

This PR bumps Protobuf to 3.15.1.

I'm new to MediaPipe and Bazel. I'm not 100% sure I generated third_party/com_google_protobuf_fixes.diff correctly but it seems to work for me.

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[johnboiles]

@googlebot I signed it!

[johnboiles]

Looks like rules_apple is only tested with Bazel 3.7 so that might also block fully moving to Bazel 4 until that's solved.