ci: stop building 6x series

It's been long enough now that things like certs have stopped
working.  These imagines have already been built and published to
Docker Hub, so this doesn't break any existing stuff.

Author: Bogdanp

.github/workflows/deploy.yml

@@ -11,7 +11,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        series: [6x, 7x, 8x]
+        series: [7x, 8x]
     steps:
     - uses: actions/checkout@master
     - run: ./build.sh ${{ matrix.series }}

.github/workflows/pr.yml

@@ -7,7 +7,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        series: [6x, 7x, 8x, snapshot]
+        series: [7x, 8x, snapshot]
     steps:
     - uses: actions/checkout@master
     - run: ./build.sh ${{ matrix.series }}