fix(policy): only allow approvals by non-author contributors (#10419)

This replaces `allow_contributor` with `allow_non_author_contributor`,
because the former allows authors to approve their own pull requests.

From https://github.com/syncthing/syncthing/pull/9818#issue-2651431707:

> This adds `allow_contributor: true` which allows approvals by contributors to the PR (*but still not the author themself*, which is a different thing).

This statement conflicts with [the policybot README](https://github.com/palantir/policy-bot/blob/c013552248d2a99911207fa5c0045428ac073fa2/README.md), which says:

> If true, the approvals of someone who has committed to the pull request are
> considered when calculating the status.
> *The pull request author is considered a contributor.*

Signed-off-by: Marcus B Spencer <marcus@marcusspencer.us>

.policy.yml

@@ -52,7 +52,7 @@ approval_rules:
         - syncthing/maintainers
     options:
       ignore_update_merges: true
-      allow_contributor: true
+      allow_non_author_contributor: true
 
   # Regular pull requests require approval by an active contributor
   - name: is approved by a syncthing contributor
@@ -62,7 +62,7 @@ approval_rules:
         - syncthing/contributors
     options:
       ignore_update_merges: true
-      allow_contributor: true
+      allow_non_author_contributor: true
 
   # Changes to some files (translations, dependencies, compatibility) do not
   # require approval if they were proposed by a contributor and have a