Use Golang 1.13 by default

Golang supports the last two released minor point versions, at the time of writing that would be 1.13 and 1.14. Omnibus uses Go 1.12, which is effectively unsupported and end-of-life'ed.

/cc @balasankarc

Components built using Go

• Gitaly - gitaly@f4c890b4
• GitLab-Shell - gitlab-shell@fcae40dd
• GitLab-Pages - gitlab-pages@550dd015
• GitLab-Workhorse - gitlab-workhorse@de2386f8
• Elasticsearch-indexer - gitlab-elasticsearch-indexer@784d9a23
• Prometheus - https://github.com/prometheus/prometheus/commit/ca424695a6e0c48dc7309c114cb1effd18181f25
• alertmanager - https://github.com/prometheus/alertmanager/commit/dea282984946d8ed9348e00f474684979c1a6cdd
• node-exporter - Go 1.13 added to test matrix after the version we use. (Balu manually tested and saw no errors on logs)
• pgbouncer-exporter - No testing, at all
• redis-exporter - https://github.com/oliver006/redis_exporter/commit/55b9cfabb601b5a71822fa396f914a07a31dcbe4
• postgres-exporter - The version we use is not tested against Go 1.13 (Balu manually tested and saw no errors on logs)
• docker-distribution-pruner -
• go-crond - Dead project
• Container Registry - container-registry@4efc0aa9