Bump etcd-druid to v0.15.3

[afritzler]

How to categorize this PR?

/area dev-productivity
/kind enhancement

What this PR does / why we need it:

This PR bumps the etcd-druid dependency to v0.15.3.

Which issue(s) this PR fixes:
Fixes #

Release note:

Bump etcd-druid dependency to v0.15.3

[gardener-prow]

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[gardener-prow]

@afritzler: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-gardener-unit	1d61e9c	link	true	/test pull-gardener-unit

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[afritzler]

/retest

[acumino]

#7248 Already tackles this. And will be merged soon.

[afritzler]

Oh I guess I missed that! I guess then we can close this one.

[afritzler]

Closing in fav of #7248