Exclude file from spotless styling to fix build

[lmossman]

What

This PR added the following json file to the repo, which is making our auto-formatting software unhappy: https://github.com/airbytehq/airbyte/blob/master/airbyte-webapp/src/test-utils/mock-data/mockDestinationDefinition.json

If someone tries to run ./gradlew format on master, the following changes are produced:

--- a/airbyte-webapp/src/test-utils/mock-data/mockDestinationDefinition.json
+++ b/airbyte-webapp/src/test-utils/mock-data/mockDestinationDefinition.json
@@ -146,7 +146,12 @@
           },
           {
             "title": "verify-full",
-            "required": ["mode", "ca_certificate", "client_certificate", "client_key"],
+            "required": [
+              "mode",
+              "ca_certificate",
+              "client_certificate",
+              "client_key"
+            ],
             "properties": {
               "mode": {
                 "enum": ["verify-full"],
@@ -218,7 +223,13 @@
           },
           {
             "title": "SSH Key Authentication",
-            "required": ["tunnel_method", "tunnel_host", "tunnel_port", "tunnel_user", "ssh_key"],
+            "required": [
+              "tunnel_method",
+              "tunnel_host",
+              "tunnel_port",
+              "tunnel_user",
+              "ssh_key"
+            ],

However, if you then try to commit these changes, the prettier pre-commit hook undoes those changes, resulting in an empty commit.

How

This PR is just a short-term fix to get the platform build back into a good state, by excluding that specific file from spotless styling so that ./gradlew format does not produce any diff for that file.