Improve the validation for a cache upstream

[ialidzhikov]

How to categorize this PR?

/kind enhancement

What this PR does / why we need it:
While working on #129 I noticed that we actually don't support upstreams with ports but our docs state that it is supported.
A Shoot reconciliation with a cache with upstream with a port (for example public.ecr.aws:443) fails with:

v1/Secret/kube-system/registry-public-ecr-aws-config-3cfbb538\": Secret \"registry-public-ecr-aws-config-3cfbb538\" is invalid: metadata.labels: Invalid value: \"public.ecr.aws:443\": a valid label must be an empty string or consist of alphanumeric characters, '-', '_' or '.', and must start and end with an alphanumeric character (e.g. 'MyValue',  or 'my_value',  or '12345', regex used for validation is '(([A-Za-z0-9][-A-Za-z0-9_.]*)?[A-Za-z0-9])?'

The above message is for Secret but the same label with the same value gets added to all resources, incl. the StatefulSet. Hence, upstream with port was not working since the initial implementation.

This PR improves the validation for the cache upstream (requires to be a valid DNS subdomain (RFC 1123)) and properly updates docs so that it is clear that upstream with port is not supported.

Which issue(s) this PR fixes:
Part of #3

Special notes for your reviewer:
N/A

Release note:

The cache upstream is now required to be a a valid DNS subdomain (RFC 1123).

[dimitar-kostadinov]

/lgtm

[gardener-prow]

LGTM label has been added.

Git tree hash: d50da5e03fd63dc58ef7051c80d4660ca4b729ab

[dimitar-kostadinov]

/retest

[ialidzhikov]

/approve

[gardener-prow]

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