Cut down pull request template text to minimum

Trialling a minimal pull request template. The Github "new PR" interface is quite overwhelming for new contributors, especially when Github adds its own signposting towards the contributing guidelines for first-time PRs - it's likely that this is leading users to see the already-filled text box as just more "wall of text", rather than something they're supposed to fill in.

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,26 +1 @@
-<!-- Thanks for contributing to Wagtail! 🎉  Please add a description below, explaining the purpose of this pull request - including the issue number of the issue you're fixing (if applicable). -->
-
-Fixes #...
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-
-_Please check the following:_
-
--   [ ] Do the tests still pass?[^1]
--   [ ] Does the code comply with the style guide?
-    -   [ ] Run `make lint` from the Wagtail root.
--   [ ] For Python changes: Have you added tests to cover the new/fixed behaviour?
--   [ ] For front-end changes: Did you test on all of Wagtail’s supported environments?[^2]
-    -   [ ] **Please list the exact browser and operating system versions you tested**:
-    -   [ ] **Please list which assistive technologies [^3] you tested**:
--   [ ] For new features: Has the documentation been updated accordingly?
-
-**Please describe additional details for testing this change**.
-
-[^1]: [Development Testing](https://docs.wagtail.org/en/latest/contributing/developing.html#testing)
-[^2]: [Browser and device support](https://docs.wagtail.org/en/latest/contributing/developing.html#browser-and-device-support)
-[^3]: [Accessibility Target](https://docs.wagtail.org/en/latest/contributing/developing.html#accessibility-targets)
+_Please describe the problem you're fixing here. Include the issue number, if applicable._