Auto-close FRs and PRs as not planned

As it is now, FRs and PRs are closed after some time of inactivity as
`completed`[1] instead of as `not_planned`.  We change that by updating to actions/stale@v6.

[1] <#8784 (comment)>

Closes GH-9681.

Author: cmb69

.github/workflows/close-stale-feature-requests.yml

@@ -15,7 +15,7 @@ jobs:
       issues: write
       pull-requests: write
     steps:
-      - uses: actions/stale@v4
+      - uses: actions/stale@v6
         with:
           days-before-close: 14
           days-before-stale: 90

.github/workflows/close-stale-prs.yml

@@ -15,7 +15,7 @@ jobs:
       issues: write
       pull-requests: write
     steps:
-      - uses: actions/stale@v4
+      - uses: actions/stale@v6
         with:
           days-before-close: 7
           days-before-stale: 60