docs: added note to review/upgrade nginx

What does this MR do?

This MR adds a note in the source installation instructions to cover the case where version of the NGINX package is lower than 1.12.1.

This MR covers the case where the nginx version is at 1.10.x, and as a result, the test check(sudo nginx -t) fails with this error: nginx: [emerg] unknown "start$temp=[filtered]$rest" variable nginx: configuration file /etc/nginx/nginx.conf test failed

The server doesn't start either until upgrading to a version greater than 1.12.1

Related issues

Found a workaround in this issue https://gitlab.com/gitlab-org/gitlab-ce/issues/38301 Closes

Author's checklist

• Apply the correct labels and milestone
• Crosslink the document from the higher-level index
• Crosslink the document from other subject-related docs
• Feature moving tiers? Make sure the change is also reflected in features.yml
• Correctly apply the product badges and tiers
• Port the MR to EE (or backport from CE): always recommended, required when the ee-compat-check job fails

Review checklist

• Your team's review (required)
• PM's review (recommended, but not a blocker)
• Technical writer's review (required)
• Merge the EE-MR first, CE-MR afterwards