MM-26584: Bump up Go patch version (#15156)

* MM-26584: Bump up Go patch version

This contains a patch fix for https://github.com/golang/go/issues/38023
which have been encountered in our load tests and some users.

I have verified on such a stuck instance and the thread dump seem to match
with what is seen on the issue. All threads stuck on runtime.futex, except one,
which is at runtime.osyield.

And of all the times load tests were run which _did not_ show this issue,
it was run with a version greater than 1.14.1, which further points to the
theory that this bug is the culprit.

* Replace docker image versions.

* Update build/README.md

Co-authored-by: Elisabeth Kulzer <elikul@elikul.de>
Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

build/README.md

@@ -18,3 +18,4 @@ In our Docker Hub Repository we have the following images:
 - `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`
+- `mattermost/mattermost-build-server:20200731_golang-1.14.6` which is based on Go 1.14.6 you can use for MM versions >= `5.26.0`