Revert "Send automated PR upon new stack release"
Revert "Send automated PR upon new stack release"

This reverts commit 730ffa8de5f4f7c7c95d8a674744a0e09fe072ba.

Lately, Elastic's release pipeline has been tagging releases from the
older 6.x series as "latest" in place of releases from the current 7.x
series. This results in weekly PRs from GitHub Actions for _downgrading_
our stack from 7.x to 6.x.

In order to avoid receiving those PRs, we disable the update workflow
entirely until the Elastic 6.x release series is marked as EOL.

Ref. elastic/elasticsearch#75436
Closes #611

deviantony/docker-elk,deviantony/docker-elk