Change ubuntu usage from latest to 20.04

[DellaBitta]

Description

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

Change the usage of GHA runners from ubuntu-latest to ubuntu-20.04.
The shift of GHA to point ubuntu-latest to `ubuntu-22.04' (citation) has caused issues

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Testing

Describe how you've tested these changes. Link any manually triggered Integration tests or CPP binary SDK Packaging Github Action workflows, if applicable.

Packaging CI -> Integration CI (GMA backoff flakes).

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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 FAILED

Requested by @DellaBitta on commit de396ba
Last updated: Tue Nov 29 07:53 PST 2022
View integration test log & download artifacts

Failures	Configs
database	[TEST] [FLAKINESS] [Android] [1/3 os: macos] [1/2 android_device: android_target] (1 failed tests)   FirebaseDatabaseTest.TestInfoConnected
firestore	[TEST] [ERROR] [Android] [2/3 os: macos ubuntu] [1/2 android_device: android_target] [TEST] [FLAKINESS] [Android] [1/3 os: windows] [1/2 android_device: android_target] (1 failed tests)   CRASH/TIMEOUT
gma	[TEST] [FLAKINESS] [iOS] [macos] [1/2 ios_device: ios_target] (1 failed tests)   FirebaseGmaTest.TestRewardedAdLoadEmptyRequest
storage	[TEST] [FLAKINESS] [Android] [2/3 os: macos ubuntu] [1/2 android_device: android_target] (1 failed tests)   CRASH/TIMEOUT

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