[SIEM][Detection Engine] Critical blocker, fixes schema accepting val…

…ues it should not (elastic#55488) (elastic#55505)

## Summary

* This fixes the schema accepting values the UI cannot handle at this point with severity. It's best to just set it to a small fixed enumeration of values.
* From feedback from people the values should have more defaults and be more consistent in the schema so gave defaults for `from`, `to`, and `interval`.
* Removed dead query examples that cannot happen because immutable cannot be set by end users anymore
* Changes the version and other sections to be integer only and not allow floats
* Added unit tests

### Checklist

Use ~~strikethroughs~~ to remove checklist items you don't feel are applicable to this PR.

~~- [ ] This was checked for cross-browser compatibility, [including a check against IE11](https://github.com/elastic/kibana/blob/master/CONTRIBUTING.md#cross-browser-compatibility)~~

~~- [ ] Any text added follows [EUI's writing guidelines](https://elastic.github.io/eui/#/guidelines/writing), uses sentence case text and includes [i18n support](https://github.com/elastic/kibana/blob/master/packages/kbn-i18n/README.md)~~

~~- [ ] [Documentation](https://github.com/elastic/kibana/blob/master/CONTRIBUTING.md#writing-documentation) was added for features that require explanation or tutorials~~

- [x] [Unit or functional tests](https://github.com/elastic/kibana/blob/master/CONTRIBUTING.md#cross-browser-compatibility) were updated or added to match the most common scenarios

~~- [ ] This was checked for [keyboard-only and screenreader accessibility](https://developer.mozilla.org/en-US/docs/Learn/Tools_and_testing/Cross_browser_testing/Accessibility#Accessibility_testing_checklist)~~

### For maintainers

~~- [ ] This was checked for breaking API changes and was [labeled appropriately](https://github.com/elastic/kibana/blob/master/CONTRIBUTING.md#release-notes-process)~~

- [x] This includes a feature addition or change that requires a release note and was [labeled appropriately](https://github.com/elastic/kibana/blob/master/CONTRIBUTING.md#release-notes-process)

x-pack/legacy/plugins/siem/server/lib/detection_engine/routes/rules/add_prepackaged_rules_route.test.ts

@@ -34,7 +34,7 @@ jest.mock('../../rules/get_prepackaged_rules', () => {
           to: 'now',
           index: ['index-1'],
           name: 'some-name',
-          severity: 'severity',
+          severity: 'low',
           interval: '5m',
           type: 'query',
           version: 2, // set one higher than the mocks which is set to 1 to trigger updates

x-pack/legacy/plugins/siem/server/lib/detection_engine/routes/rules/get_prepackaged_rule_status_route.test.ts

@@ -33,7 +33,7 @@ jest.mock('../../rules/get_prepackaged_rules', () => {
           to: 'now',
           index: ['index-1'],
           name: 'some-name',
-          severity: 'severity',
+          severity: 'low',
           interval: '5m',
           type: 'query',
           version: 2, // set one higher than the mocks which is set to 1 to trigger updates