Revert "IS-364: Bump Golang CI. (#14123)" (#14133)

This reverts commit 81d6ae2207.

build/README.md

@@ -17,4 +17,3 @@ In our Docker Hub Repository we have the following images:
 - `mattermost/mattermost-build-server:dec-7-2018` which is based on Go 1.11 you can use for MM versions <= `5.8.0`
 - `mattermost/mattermost-build-server:feb-28-2019` which is based on Go 1.12 you can use for MM versions >= `5.9.0` <= `5.15.0`
 - `mattermost/mattermost-build-server:sep-17-2019` which is based on Go 1.12.9 you can use for MM versions >= `5.16.0`
-- `mattermost/mattermost-build-server:20200322_golang-1.14.1` which is based on Go 1.14.1 you can use for MM versions >= `5.24.0`