Use strict = True for mypy_bootstrap.ini (#18076)

We almost had all strict flags set anyway, so many will just work.
If not, it would be easier to turn something off.

mypy_bootstrap.ini

@@ -1,16 +1,6 @@
 [mypy]
-disallow_untyped_calls = True
-disallow_untyped_defs = True
-disallow_incomplete_defs = True
-check_untyped_defs = True
-disallow_subclassing_any = True
-warn_no_return = True
-strict_optional = True
-no_implicit_optional = True
-disallow_any_generics = True
-disallow_any_unimported = True
-warn_redundant_casts = True
-warn_unused_configs = True
+strict = True
+warn_unused_ignores = False
 show_traceback = True
 always_true = MYPYC