fix: show client secret input for PKCE auth code flow

PKCE and Client Secrets are allowed to coexist and neither is designed
as a replacement for the other. [1] It is wrong to assume that a client
secret must not or cannot be used in combination with PKCE. Quite the
opposite, when possible both PKCE and client secret should be used. [2]
So the premises of #6290 and #8146 are not correct.

Admittedly, for users of the PKCE mechanism WITHOUT a client secret it
might be a minor nuisance to see the client secret input in the Swagger
UI. But they can just leave it empty. On the other hand, for users of
the PKCE mechanism WITH a client secret it is more than just a nuisance
if the client secret input is not shown. The Swagger UI becomes unusable
for them (unless they've set a default value for the client secret,
which will be used hiddenly without being shown to the user).

Therefore the right course of action for now would be to revert #7438 to
show the client secret input always regardless of PKCE. In the future a
new flag could be introduced to hide the client secret input regardless
of the PKCE flag.

[1] https://oauth.net/2/pkce/
[2] https://www.oauth.com/oauth2-servers/pkce/

src/core/components/auth/oauth2.jsx

@@ -212,7 +212,7 @@ export default class Oauth2 extends React.Component {
         }
 
         {
-          ( (flow === AUTH_FLOW_APPLICATION || flow === AUTH_FLOW_ACCESS_CODE && !isPkceCodeGrant || flow === AUTH_FLOW_PASSWORD) && <Row>
+          ( (flow === AUTH_FLOW_APPLICATION || flow === AUTH_FLOW_ACCESS_CODE || flow === AUTH_FLOW_PASSWORD) && <Row>
             <label htmlFor="client_secret">client_secret:</label>
             {
               isAuthorized ? <code> ****** </code>