Patch flatbuffers to fix a warning that occurs with some compiler versions

[AlmostMatt]

Description

Add a patch to the flatbuffers version that we are pinned to that fixes an unused-variable warning.
Without this, after updating to xcode 14.3 a newer c compiler fails the build due to this warning about flatbuffers source.

In the flatbuffers repo this unused variable is not removed until much later when the relevant code is basically completely rewritten. So pinning to a newer version of flatbuffers can have other side effects. (Including requiring a newer version of cmake)

The content of the cmake/external/flatbuffers file is basically copied from the similar websockets file which also applies a patch.

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Testing

Verified that local builds fail without this patch and succeed with this patch.

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Type of Change

Place an x the applicable box:

• Bug fix. Add the issue # below if applicable.
• New feature. A non-breaking change which adds functionality.
• Other, such as a build process or documentation change.

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Notes

• Bug fixes and feature changes require an update to the Release Notes section of release_build_files/readme.md.
• Read the contribution guidelines CONTRIBUTING.md.
• Changes to the public API require an internal API review. If you'd like to help us make Firebase APIs better, please propose your change in a feature request so that we can discuss it together.

[github-actions]

Integration test with FLAKINESS (succeeded after retry)

Requested by @AlmostMatt on commit a395148
Last updated: Mon May 15 14:59 PDT 2023
View integration test log & download artifacts

Failures	Configs
firestore	[TEST] [FLAKINESS] [Android] [1/3 os: ubuntu] [1/2 android_device: android_target] (1 failed tests)   TransactionTest.TestCancellationOnError [TEST] [FLAKINESS] [Android] [2/3 os: ubuntu macos] [1/2 android_device: emulator_ftl_target] (1 failed tests)   CRASH/TIMEOUT

Add flaky tests to go/fpl-cpp-flake-tracker