Update adopting strict docs for 1.0 (#14865)

docs/source/existing_code.rst

@@ -183,15 +183,14 @@ An excellent goal to aim for is to have your codebase pass when run against ``my
 This basically ensures that you will never have a type related error without an explicit
 circumvention somewhere (such as a ``# type: ignore`` comment).
 
-The following config is equivalent to ``--strict`` (as of mypy 0.990):
+The following config is equivalent to ``--strict`` (as of mypy 1.0):
 
 .. code-block:: text
 
    # Start off with these
    warn_unused_configs = True
    warn_redundant_casts = True
    warn_unused_ignores = True
-   no_implicit_optional = True
 
    # Getting these passing should be easy
    strict_equality = True