revert: roll back .NET 10 upgrade — Flex stamp missing runtime

Reverts:
- 8dacce5c "chore(backend): bump optional NuGet patches + fix SYSLIB0057"
- 2f68fe69 "chore(backend): upgrade Functions from .NET 8 to .NET 10 isolated"

After deploy succeeded and the Flex Consumption stack was switched to
.NET 10 Isolated in the portal, the dotnet-isolated worker died on every
boot with: dotnet exited with code 150 (FrameworkMissingFailure) and
"Exceeded language worker restart retry count for runtime:dotnet-isolated".
The deployed package was correct (TFM net10.0, FrameworkReference 10.0.0,
all package bumps applied) and the runtime stack property was set to
dotnet-isolated 10.0, but the .NET 10 runtime is not present on the
Flex Consumption stamp in West Europe even though Microsoft lists
DOTNET-ISOLATED|10.0 as supported. Restart and stop+start did not help.

Recovery sequence:
1. Stack switched back to .NET 8 Isolated (manual portal action).
2. Re-deployed last known-good net8.0 commit 637bd9a1 via temp branch
   rollback-net8 (now safe to delete).
3. /api/health back to 200.

Follow-ups:
- Re-apply security fixes (DataProtection 10.0.7 pin GHSA-9mv3-2cwr-p262,
  SYSLIB0057 X509CertificateLoader migration) on net8.0 in a separate PR.
- Open Microsoft support: ".NET 10 Isolated worker on Flex Consumption
  WestEurope fails with FrameworkMissingFailure despite stack listed as
  supported."

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.github/workflows/deploy-backend.yml

@@ -16,11 +16,10 @@ jobs:
         with:
           creds: ${{ secrets.AZURE_CREDENTIALS }}
 
-      - name: Setup .NET 10
+      - name: Setup .NET 8
         uses: actions/setup-dotnet@v4
         with:
-          dotnet-version: '10.0.x'
-          dotnet-quality: 'ga'
+          dotnet-version: '8.0.x'
 
       - name: Restore dependencies
         run: dotnet restore src/Backend/AutopilotMonitor.Functions/AutopilotMonitor.Functions.csproj