[release/3][BACKPORT] Bump fluent-bit to 0.15.4

[d2iq-mergebot]

This is a backport of the following PR:

#1039

What type of PR is this?

Chore

What this PR does/ why we need it:

Update the fluent-bit chart to 0.15.4, and the appversion to 1.7.3.

Which issue(s) this PR fixes:

no issue

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

Update to the latest minor version just to keep up with upstream. The bugfixes listed should have no effect on our default configurations.

https://fluentbit.io/announcements/v1.7.0/
https://fluentbit.io/announcements/v1.7.1/
https://fluentbit.io/announcements/v1.7.2/
https://fluentbit.io/announcements/v1.7.3/

Checklist

• The commit message explains the changes and why are needed.
• The code builds and passes lint/style checks locally.
• The relevant subset of integration tests pass locally.
• The core changes are covered by tests.
• The documentation is updated where needed.

[d2iq-mergebot]

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@mesosphere-mergebot backport  

[alejandroEsc]

@joejulian does the release3 branch have a fix to the tests that is missing?

[joejulian]

This is timing out deploying. Needs manual testing.