Update deployment page to include a set line for using a PG13 image
Summary
For the 6.0 release we aren't forcing users to upgrade to PG13, but we do want to start to have fresh installs default to PG13. For the charts we can't change the default image version on folks without forcing upgrade, so we need to instead work on documentation.
We should update the default install script instructions in https://gitlab.com/gitlab-org/charts/gitlab/-/blob/master/doc/installation/deployment.md#deploy-using-helm to include a line that sets the postgres version to 13
and also include a note about postgres versions in https://gitlab.com/gitlab-org/charts/gitlab/-/blob/master/doc/installation/deployment.md#postgresql