[Screenshotting Diagnostics] Change endpoint for checking browser issues to GET

[tsullivan]

In order to incorporate the screenshotting diagnostics endpoint into internal Support diagnostics tooling, the endpoint for the diagnostics tool must be switched from POST to GET.

Current:

curl 'http://localhost:5601/api/reporting/diagnose/browser' \
  -X 'POST' \
  -H 'Content-Type: application/json' \
  -H 'kbn-version: 8.8.2' \' 

Related [internal]: elastic/support-diagnostics#676

[elasticmachine]

Pinging @elastic/appex-sharedux (Team:SharedUX)

[vadimkibana]

• Screenshotting would launch a blank page and respond with binary good/problem.
• If error happens, would return detailed error.