fix(relay): Update projects.mdx (#14606)

- Remove reference to the no longer needed `slug`
- Replace the hard-coded private DSN keys with the new dropdown (that is
already used elsewhere).

Author: tobias-wilfert

docs/product/relay/projects.mdx

@@ -27,10 +27,9 @@ fields. The minimal configuration **must** contain the following fields:
 
 ```json
 {
-  "slug": "my-project",
   "publicKeys": [
     {
-      "publicKey": "<DSN_KEY>",
+      "publicKey": "___PUBLIC_KEY___",
       "isEnabled": true
     }
   ],
@@ -40,25 +39,8 @@ fields. The minimal configuration **must** contain the following fields:
 }
 ```
 
-<Alert level="warning" title="Public Key">
-
-The public key (`<DSN_KEY>`) is the key of the project's DSN and is unrelated to the Relay public key used for Relay registration.
-
-</Alert>
-
 ## Basic Options
 
-`slug`
-
-The short name of the project, displayed in Sentry. This value is currently
-required for Relay to accept events.
-
-```json
-{
-  "slug": "my-project"
-}
-```
-
 `disabled`
 
 Whether the project is disabled. If set to `true`, the Relay will drop all
@@ -79,7 +61,7 @@ that key should be accepted.
 {
   "publicKeys": [
     {
-      "publicKey": "12345abcdb1e4c123490ecec89c1f199",
+      "publicKey": "___PUBLIC_KEY___",
       "isEnabled": true
     }
   ]