ci: bump pinned pnpm version from 10 to 11

ether/etherpad-lite's package.json declares packageManager: pnpm@11.0.6.
pnpm/action-setup@v6 errored with ERR_PNPM_BAD_PM_VERSION when version
was pinned to 10 because the action saw the conflict with the core's
packageManager field. Bumping the pin to 11 resolves the mismatch.

Author: JohnMcLear

.github/workflows/backend-tests.yml

@@ -33,7 +33,7 @@ jobs:
       - uses: pnpm/action-setup@v6
         name: Install pnpm
         with:
-          version: 10
+          version: 11
           run_install: false
       - name: Get pnpm store directory
         shell: bash

.github/workflows/frontend-tests.yml

@@ -23,7 +23,7 @@ jobs:
       - uses: pnpm/action-setup@v6
         name: Install pnpm
         with:
-          version: 10
+          version: 11
           run_install: false
       - name: Get pnpm store directory
         shell: bash

.github/workflows/npmpublish.yml

@@ -34,7 +34,7 @@ jobs:
       - uses: pnpm/action-setup@v6
         name: Install pnpm
         with:
-          version: 10
+          version: 11
           run_install: false
       - name: Get pnpm store directory
         shell: bash