Add support for PostgreSQL 11 to gitlab-ctl pg-upgrade

This MR adds support for PostgreSQL 11 to gitlab-ctl pg-upgrade.

pg-upgrade and revert-pg-upgrade commands behave as follows:

• gitlab-ctl pg-upgrade always upgrades to PostgreSQL 11 regardless of the running version.
• If upgrade procedure fails it will be reverted back to the running version.
• gitlab-ctl revert-pg-upgrade downgrades to the version that was running before upgrading with gitlab-ctl pg-upgrade. If it can't find the previous version (which is stored in BASE_PATH/embedded/postgresql/version.old after upgrade) it will attempt to revert to PostgreSQL 10. always downgrades to PostgreSQL 10 regardless of what was the running version before upgrading. This imposes a limitation when the upgrade was done from 9.6.

This MR has been verified in the following scenarios:

• Successful upgrade from 9.6 to 11
• Successful upgrade from 10 to 11
• Erroneous upgrade attempt from 9.6 to 11
• Erroneous upgrade attempt from 10 to 11
• Downgrade from 11 to 10
• Downgrade from 11 to 9.6

NOTE: This MR lacks the proper documentation.

Closes #4973 (closed)