Set Custom SLIs for Web and API endpoints on Pipeline Authoring

Following the steps from gitlab-com/gl-infra/scalability#1315, identify the endpoints owned by grouppipeline authoring that would require a latency target > 1 second, and set a new target duration for those endpoints.

Web/API Endpoints identified that require custom targets

Endpoint	Type	New target duration	User Impact	Custom SLI set?
POST /api/projects/:id/ci/lint	API	5 seconds	No
GET /api/projects/:id/ci/lint	API	5 seconds	No
GET /api/:version/projects/:id/pipelines/:pipeline_id/variables	API	5 seconds	No
Projects::Ci::LintsController#create.json	Web	5 seconds	Yes
Projects::Ci::PipelineEditorController#show	Web	5 seconds	Yes
Projects::VariablesController#update.json	Web	5 seconds	Yes
Projects::VariablesController#show.json	Web	5 seconds	Yes
Groups::VariablesController#show.json	Web	5 seconds	No

Note that the goal for completing this exercise is by Dec 1 2021.

The related MR to remove ignored_components is gitlab-com/runbooks!4160 (merged)