fix: helm - nginx tenants proxy headers

[a5r0n]

What this PR does / why we need it:

When using basic auth, we set the X-Scope-OrgID to the user name with proxy_set_header,
but according to the nginx docs:

Allows redefining or appending fields to the request header passed to the proxied server.
The value can contain text, variables, and their combinations.
These directives are inherited from the previous configuration level if and only if there are no proxy_set_header directives defined on the current level.

So whenever we set proxy headers on the location block level, we should also include the X-Scope-OrgID set.

Which issue(s) this PR fixes:
Fixes #16938
Should close #7153 #11915

Special notes for your reviewer:

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Documentation added
• Tests updated
• Title matches the required conventional commits format, see here
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