chore(ci): tweak backend filters (#17134)

The existing backend filters get triggered even on frontend-only or
docs-only changes, which should not be the case. The reason for this
seems to be the fact that each filter line is ORed rather than ANDed.
To remedy this, we put all the filters on the same line.

I tried the filter out in a REPL
(https://runkit.com/blakepettersson/65c3daba99653f0008c74eda). This is
a filter using picomatch (the same library `dorny/paths-filter` uses).

Signed-off-by: Blake Pettersson <[email protected]>

.github/workflows/ci-build.yaml

@@ -33,10 +33,10 @@ jobs:
       - uses: dorny/paths-filter@4512585405083f25c027a35db413c2b3b9006d50 # v2
         id: filter
         with:
+          # Any file which is not under docs/, ui/ or is not a markdown file is counted as a backend file
           filters: |
             backend:
-              - '!(ui/**)'
-              - '!(**/*.md)'
+              - '!(ui/**|docs/**|**.md|**/*.md)'
             frontend:
               - 'ui/**'
   check-go: