build: remove typescript 3.6 and 3.7 support (#36329)

Remove TypeScript 3.6 and 3.7 support from Angular along with tests that
ensure those TS versions work.

BREAKING CHANGE: typescript 3.6 and 3.7 are no longer supported, please
update to typescript 3.8

PR Close #36329

tools/ts-api-guardian/package.json

@@ -17,7 +17,7 @@
     "test": "test"
   },
   "peerDependencies": {
-    "typescript": "~3.7.4"
+    "typescript": "~3.8.3"
   },
   "dependencies": {
     "chalk": "^2.3.1",
@@ -33,7 +33,7 @@
     "chai": "^4.1.2",
     "jasmine": "^3.1.0",
     "source-map-support": "^0.5.9",
-    "typescript": "~3.7.4"
+    "typescript": "~3.8.3"
   },
   "repository": {},
   "keywords": [