Bump dartdoc to 7.0.1 and clean-up (#136975)

These warnings/errors have been removed from dartdoc. Removing them from the flutter config as well.

Also fixes CI to actually test doc generation for all documented packages and on dartdoc config changes.
diff --git a/dartdoc_options.yaml b/dartdoc_options.yaml
index f73d457..545194e 100644
--- a/dartdoc_options.yaml
+++ b/dartdoc_options.yaml
@@ -14,13 +14,12 @@
       command: ["bin/cache/dart-sdk/bin/dart", "pub", "global", "run", "snippets", "--output-directory=doc/snippets", "--type=dartpad"]
       description: "Creates full application sample code documentation output from embedded documentation samples and displays it in an embedded DartPad."
   errors:
-    # Default errors of dartdoc:
+    ## Default errors of dartdoc:
     - duplicate-file
     - invalid-parameter
-    - no-defining-library-found
     - tool-error
     - unresolved-export
-    # Warnings that are elevated to errors:
+    ## Warnings that are elevated to errors:
     - ambiguous-doc-reference
     - ambiguous-reexport
     - broken-link
@@ -31,10 +30,12 @@
     - no-canonical-found
     - no-documentable-libraries
     - no-library-level-docs
-    - not-implemented
     - orphaned-file
     - reexported-private-api-across-packages
-    # - unknown-directive  # Disabled due to https://github.com/dart-lang/dartdoc/issues/2353
     - unknown-file
     - unknown-macro
     - unresolved-doc-reference
+    ## Ignores that are elevated to errors:
+    # - type-as-html # not yet tested
+    # - missing-constant-constructor # not yet tested
+    # - missing-code-block-language # not yet tested