Update profiles schema to draft 2020-12 (#10280)

Update profiles schema to draft 2020-12 because as mentioned in microsoft/vscode#98724 (comment), OpenAPI Specification 3.1 defines using JSON Schema 2020-12 and VS Code already has early implementation around it.

Basically, this just gets rid of the following error shown by VS Code when editing the settings.json file
```
Draft 2019-09 schemas are not yet fully supported.
```

## PR Checklist
* [x] CLA signed. If not, go over [here](https://cla.opensource.microsoft.com/microsoft/Terminal) and sign the CLA
* [x] Schema updated.

doc/cascadia/profiles.schema.json

@@ -1,6 +1,6 @@
 {
   "$id": "https://github.com/microsoft/terminal/blob/main/doc/cascadia/profiles.schema.json",
-  "$schema": "https://json-schema.org/draft/2019-09/schema#",
+  "$schema": "https://json-schema.org/draft/2020-12/schema#",
   "title": "Microsoft's Windows Terminal Settings Profile Schema",
   "definitions": {
     "KeyChordSegment": {