[release/4][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/v1.6.9/
https://fluentbit.io/announcements/v1.6.10/
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.
  Before merging, conflicts in the following files need to be resolved:
  addons/fluentbit/fluentbit.yaml

[d2iq-mergebot]

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@mesosphere-mergebot backport  

[joejulian]

This will not ship with 4.0.0 as it would require soaking and missed the cutoff to have time to do so. It will ship in release/next.