Finish the Linux 5.9 branch

The Linux 5.9 kernel branch has reached its end-of-life and will not
receive any new update. Therefore, we close of this branch.

.github/workflows/watch.yml

@@ -11,7 +11,7 @@ jobs:
     runs-on: [ubuntu-latest]
     strategy:
       matrix:
-        branch: ['v5.9.x']
+        branch: []
     steps:
     - name: Checkout Sources
       uses: actions/checkout@v2

README.md

@@ -5,7 +5,7 @@ Custom Linux kernels for Promox VE 6.
 1. Linux 5.6 (EOL)
 2. Linux 5.7 (EOL)
 3. Linux 5.8 (EOL)
-4. Linux 5.9
+4. Linux 5.9 (EOL)
 
 #### Flavors
 1. Proxmox