Remove centos6 release

Centos 6 is missing ghc 8.4.4 support, see
https://github.com/commercialhaskell/stack/issues/4161

The static binary release for linux can be used instead.

.circleci/config.yml

@@ -255,9 +255,6 @@ jobs:
             psql "postgres:///postgrest_test" -f test/fixtures/privileges.sql
             test/memory-tests.sh
 
-  centos6:
-    <<: *build-distro-bin
-
   centos7:
     <<: *build-distro-bin
 
@@ -317,18 +314,6 @@ workflows:
           filters:
             tags:
               only: /v[0-9]+(\.[0-9]+)*/
-      - centos6:
-          requires:
-            - build-test-9.4
-            - build-test-9.6
-            - build-test-10
-            - build-test-11
-            - build-prof-test
-          filters:
-            tags:
-              only: /v[0-9]+(\.[0-9]+)*/
-            branches:
-              ignore: /.*/
       - centos7:
           requires:
             - build-test-9.4
@@ -367,7 +352,6 @@ workflows:
               ignore: /.*/
       - release:
           requires:
-            - centos6
             - centos7
             - ubuntu
             - ubuntui386