Update obs-onboarding cypress CI config (elastic#186926)

- removes parallelism: 2 from step definition. The test suites are not
sharded.
- Updates the path used to trigger a test run. The previous path is out
of date.

.buildkite/pipelines/pull_request/observability_onboarding_cypress.yml

@@ -8,7 +8,6 @@ steps:
       - build
       - quick_checks
     timeout_in_minutes: 120
-    parallelism: 2
     retry:
       automatic:
         - exit_status: '-1'

.buildkite/scripts/pipelines/pull_request/pipeline.ts

@@ -83,7 +83,7 @@ const getPipeline = (filename: string, removeSteps = true) => {
 
     if (
       (await doAnyChangesMatch([
-        /^x-pack\/plugins\/observability_onboarding/,
+        /^x-pack\/plugins\/observability_solution\/observability_onboarding/,
         /^x-pack\/plugins\/fleet/,
       ])) ||
       GITHUB_PR_LABELS.includes('ci:all-cypress-suites')