add pr templates with extension upgrade checklist (#706)

Author: egor-romanov

.github/PULL_REQUEST_TEMPLATE/default.md

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+## What kind of change does this PR introduce?
+
+Bug fix, feature, docs update, ...
+
+## What is the current behavior?
+
+Please link any relevant issues here.
+
+## What is the new behavior?
+
+Feel free to include screenshots if it includes visual changes.
+
+## Additional context
+
+Add any other context or screenshots.

.github/PULL_REQUEST_TEMPLATE/extension_upgrade.md

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+## What kind of change does this PR introduce?
+
+- upgrade _extension_ from _v0.0.0_ to _v0.0.0_
+
+## Additional context
+
+Add any other context or screenshots.
+
+## Action Items
+
+- [ ] **New extension releases** were Checked for any breaking changes
+- [ ] **Extensions compatibility** Checked
+    * Proceed to [extensions compatibility testing](#extensions-compatibility-testing), mark as done after everything is completed
+- [ ] **Backup and Restore** Checked
+    * Proceed to [backup testing](#backup-testing) while extensions are enabled
+        - After every restore, re-run the tests specified at point [3.1](#extensions-compatibility-testing)
+
+### Extensions compatibility testing
+
+1. Enable every extension
+    1. Check Postgres’ log output for any error messages while doing so
+        1. This might unearth incompatibilities due to unsupported internal functions, missing libraries, or missing permissions
+2. Disable every extension
+    1. Check Postgres’ log output for any cleanup-related error messages
+3. Re-enable each extension
+    1. Run basic tests against the features they offer, e.g.:
+        1. `pg_net` - execute HTTP requests
+        2. `pg_graphql` - execute queries and mutations
+        3. …to be filled in
+
+### Backup Testing
+
+Follow the testing steps steps for all the following cases:
+
+- Pause on new Postgres version, restore on new Postgres version
+- Pause on older Postgres version, restore on new Postgres version
+- Run a single-file backup backup, restore the backup
+
+#### Testing steps
+
+1. Generate dummy data 
+    * the ‘Countries’ or ‘Slack clone’ SQL editor snippets are decent datasets to work with, albeit limited
+2. Save a db stats snapshot file
+    * Do this by running `supa db-stats gather -p <project_ref>`
+3. Backup the database, through pausing the project, or otherwise
+4. Restore the backup, through unpausing the project or cli
+5. Check the data has been recovered successfully
+    1. Visual checks/navigating through the tables works
+    2. Run `supa db-stats verify` against the project and the previously saved file

.github/pull_request_template.md

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+Please go the the `Preview` tab and select the appropriate sub-template:
+
+* [Default](?expand=1&template=default.md)
+* [Extension Upgrade](?expand=1&template=extension_upgrade.md)