Update GitHub PR template

[axw]

Motivation/summary

As the vast majority of PRs come from Elastic employees, let's optimise the PR template a little. We have a CONTRIBUTING.md for everyone else, we don't need to duplicate it in the template.

I've attempted to simplify the template into a few reminders, and minimise the amount of process involved in creating a PR. We can and should keep each other honest in reviews, but the onus is still on PR authors.

Checklist

• I have signed the Contributor License Agreement.
  - [ ] I have updated CHANGELOG.asciidoc

I have considered changes for:
- [ ] documentation
- [ ] logging (add log lines, choose appropriate log selector, etc.)
- [ ] metrics and monitoring (create issue for Kibana team to add metrics to visualizations, e.g. Kibana#44001)
- [ ] automated tests (add tests for the code changes, all unit tests pass locally)
- [ ] telemetry
- [ ] Elasticsearch Service (https://cloud.elastic.co)
- [ ] Elastic Cloud Enterprise (https://www.elastic.co/products/ece)
- [ ] Elastic Cloud on Kubernetes (https://www.elastic.co/elastic-cloud-kubernetes)

How to test these changes

Create a PR

Related issues

None

[apmmachine]

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Build stats

• Build Cause: Branch indexing

• Start Time: 2020-12-17T02:50:10.694+0000

• Duration: 43 min 43 sec

Test stats 🧪

Test	Results
Failed	0
Passed	4613
Skipped	124
Total	4737

Steps errors

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Run Window tests

• Took 9 min 16 sec . View more details on here

Compress

• Took 0 min 0 sec . View more details on here
• Description: tar --exclude=coverage-files.tgz -czf coverage-files.tgz coverage

Compress

• Took 0 min 0 sec . View more details on here
• Description: tar --exclude=system-tests-linux-files.tgz -czf system-tests-linux-files.tgz system-tests

Test Sync

• Took 3 min 8 sec . View more details on here
• Description: ./.ci/scripts/sync.sh