Update SAST and GitLab Advanced SAST docs to explain GitLab Advanced SAST is enabled by default

Proposal

As part of Enable GitLab Advanced SAST by default (&15145) • Unassigned • Needs attention, we need to update the following docs to explain that GitLab Advanced SAST is now executed by default:

• Static Application Security Testing (SAST)
• GitLab Advanced SAST

Implementation Plan

Use Draft: Document GLAS enabled by default (!182227 - closed) • Adam Cohen • 18.0 as a starting point.

Make the following docs changes:

1. Static Application Security Testing (SAST)

   1. Remove reference to GITLAB_ADVANCED_SAST_ENABLED from Analyzer Settings.

   2. Document differences between available commandline options for semgrep and gitlab-advanced-sast.

      For example, semgrep supports "--max-memory", "--max-target-bytes", "--timeout", whereas gitlab-advanced-sast supports "--sca-output-path", "--max-memory".

   3. Explain language coverage between semgrep and GitLab Advanced SAST. For example, semgrep handles the languages that GitLab Advanced SAST doesn't yet support.

2. GitLab Advanced SAST

   1. Remove references to GITLAB_ADVANCED_SAST_ENABLED from Enable Advanced SAST scanning.

   2. Explain the increase in scan duration and potential out-of-memory errors, and how to troubleshoot them.