Update the Unity versions being used

[a-maurice]

Description

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

u3d recently released a new version that is causing problems with our use case. Until we can fix it, pin to the previously released version.

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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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[sunmou99]

I believe this is the optional solution for now.
Because as you mentioned:

1. new version of u3d has a issue: DragonBox/u3d#421
2. older version of u3d requires us to use a newer version: https://github.com/firebase/firebase-unity-sdk/runs/6476560371?check_suite_focus=true#step:9:2206

[github-actions]

❌  Integration test FAILED

Requested by @a-maurice on commit e9f597b
Last updated: Wed May 18 14:28 PDT 2022
View integration test log & download artifacts

Failures	Configs
firestore	[TEST] [ERROR] [2019] [macos] [1/5 Platform(s): Android] [1/2 Test Device(s): android_target] [TEST] [FAILURE] [2019] [macos] [2/5 Platform(s): Android ubuntu] [1/2 Test Device(s): android_target] [TEST] [FLAKINESS] [2019] [macos] [1/5 Platform(s): iOS] [1/2 Test Device(s): ios_target]
messaging	[TEST] [ERROR] [2019] [macos] [2/5 Platform(s): Android iOS] [All 2 Test Device(s)]
storage	[BUILD] [ERROR] [2019] [macos] [All 3 Platform(s)]

[sunmou99]

LGTM