Set the apdex SLO for codesuggestions to 99.9%

Set the apdex SLO for codesuggestions to 99.9%

This tightens the apdex SLO for code suggestions. Since the move to only 3rd party models, we have met this apdex consistently.

For https://gitlab.com/gitlab-com/gl-infra/reliability/-/issues/23939

SLO analysis: https://dashboards.gitlab.net/d/alerts-component_slo_apdex/alerts-global-sli-metrics-apdex-slo-analysis?orgId=1&var-PROMETHEUS_DS=Global&var-environment=gprd&var-type=code_suggestions&var-stage=main&var-component=ingress&var-proposed_slo=0.999&from=1687784575341&to=1688389375341

The +100% markings there are likely caused by gitlab-com/gl-infra/scalability#2319 (closed)