[prometheus-json-exporter] Fix scrapeTimeout validation

[CoolDuke]

What this PR does / why we need it

The current implementation checks for the non-existent value serviceMonitor.scrapeTimeout which leads to a fallback to the default scrape timeout configured in Prometheus (usually 10s).

The PR ensures that the configured scrapeTimeout value for a target is used or if not defined the default value is used.

Which issue this PR fixes

none

Special notes for your reviewer

Checklist

• [x] DCO signed
• [x] Chart Version bumped
• [x] Title of the PR starts with chart name (e.g. [prometheus-couchdb-exporter])