Add reason for unauthorized in jira connect API

What does this MR do and why?

Adds a reason to 401 unauthorized responses from api/v4/integrations/jira_connect/subscriptions to avoid confusion around what authentication method has failed.

This API endpoint is behind the jira_connect_oauth feature flag.

Related to: #362338 (closed)

How to set up and validate locally

1. In rails console enable the invite modal

   Feature.enable(:jira_connect_oauth)

2. Request the endpoint with a wrong auth token:

    curl --location --request POST 'http://127.0.0.1:3000/api/v4/integrations/jira_connect/subscriptions' \
    --header 'accept: application/json' \
    --header 'Authorization: Bearer invalid_token' \
    --header 'Content-Type: application/json' \
    --data-raw '{
        "jwt": "invalid_jwt",
        "namespace_path": "flightjs"
    }'

3. This should result in { "message": "401 Unauthorized" }
4. Got to http://localhost:3000/-/profile/personal_access_tokens
5. Create a new token with the api scope
6. Copy the token
7. Request the endpoint with the new, valid auth token but invalid JWT

   curl --location --request POST 'http://127.0.0.1:3000/api/v4/integrations/jira_connect/subscriptions' \
   --header 'accept: application/json' \
   --header 'Authorization: Bearer {{ ADD YOUR VALID TOKEN HERE }' \
   --header 'Content-Type: application/json' \
   --data-raw '{
       "jwt": "invalid_jwt",
       "namespace_path": "flightjs"
   }'

8. This should result in { "message": "401 Unauthorized - JWT authentication failed" }

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