Fix CodeQL issues for checked-in yarn binaries (#1017)

## Description

This PR removes unnecessary `.yarn` folders from the repo and prevents
them from being checked-in / scanned by CodeQL in the future.

### Why
Having yarn binaries checked-in is a security risk and CodeQL complains
about issues in the scripts (which we can't fix, or even see, as they're
obfuscated as binary files).

Removing the `.yarn` folder for older samples is not a problem, but
newer samples which use Yarn 3, such as `NativeModuleSample/cpp-lib`,
need the Yarn 3 binaries checked-in.

Unfortunately there's no way to get around this. Yarn refuses to
acknowledge this anti-pattern and security issue (see
yarnpkg/yarn#7741) and the "solution" to
require users to install Yarn 3 via `corepack` does not work in GitHub
CI (see actions/setup-node#480). I've tried
every workaround combination in these two issues and none work.

Closes #1016 

## Screenshots
N/A

###### Microsoft Reviewers: [Open in
CodeFlow](https://microsoft.github.io/open-pr/?codeflow=https://github.com/microsoft/react-native-windows-samples/pull/1017)

Author: jonthysell

.gitignore

@@ -14,4 +14,5 @@ website/i18n/*
 
 # This is not a mono repo, ignore files from people accidentally running yarn in the wrong places
 /yarn.lock
-/samples/yarn.lock
+/samples/yarn.lock
+/samples-old/yarn.lock

CodeQL.yml

@@ -5,3 +5,5 @@ path_classifiers:
   docs:
     - "**/docs"
     - "**/website/versioned_docs"
+  library:
+    - "**/.yarn"