Let dependabot consider all references of the alpine and pause images when opening PRs to bump their version

[plkokanov]

How to categorize this issue?

/area dev-productivity
/kind enhancement

What would you like to be added:
Currently, dependabot only opens PRs to update the alpine version used to build the rsyslog-relp-echo-server container:

gardener-extension-shoot-rsyslog-relp/Dockerfile

Line 39 in f78f9da

FROM alpine:3.20.3 AS shoot-rsyslog-relp-echo-server

However, alpine is also used in other places: in the images.yaml file, and the lifecycle integration test. This means that developers need to manually bump these versions which can be inconvenient.

Additionally, the pause image in the images.yaml file should also be automatically handled by dependabot.

Why is this needed:
To automate version bumps of all alpine and pause image references and save dev effort.