Add C++ fixes in 9.2.0 to readme.

[jonsimantov]

Description

Provide details of the change, and generalize the change in the PR title above.

Fixes from C++ SDK reflected in Unity SDK.

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Testing

Describe how you've tested these changes.

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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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[github-actions]

❌  Integration test FAILED

Requested by @firebase-workflow-trigger[bot] on commit 2a1f9e7
Last updated: Mon Jul 11 17:22 PDT 2022
View integration test log & download artifacts

Failures	Configs
analytics	[TEST] [ERROR] [2019] [macos] [1/5 Platform(s): iOS] [1/3 Test Device(s): simulator_target]
firestore	[BUILD] [ERROR] [2019] [macos] [1/3 Platform(s): Android]
functions	[TEST] [ERROR] [2019] [macos] [1/5 Platform(s): iOS] [1/3 Test Device(s): simulator_target]
installations	[TEST] [ERROR] [2019] [macos] [1/5 Platform(s): iOS] [1/3 Test Device(s): simulator_target]
messaging	[TEST] [ERROR] [2019] [macos] [1/5 Platform(s): iOS] [1/3 Test Device(s): ios_target] [TEST] [FAILURE] [2019] [macos] [1/5 Platform(s): iOS] [1/3 Test Device(s): simulator_target]
storage	[TEST] [FAILURE] [2019] [macos] [4/5 Platform(s): windows iOS ubuntu macos] [1/3 Test Device(s): simulator_target] [TEST] [FLAKINESS] [2019] [macos] [2/5 Platform(s): Android iOS] [All 2 FTL Devices]