fix(crons): Correct description of start_time (#383)

This actually is NOT when relay kafka produces the message into kafka
(we can use the headers timestamp for that, since w're configured for
LogAppendTime)

Here's where this is actually coming from

https://github.com/getsentry/relay/blob/671bdf354b1fdc401d1a1c39430b7b1da3e70cec/relay-server/src/services/store.rs#L845

Where the `start_time` here actually comes from the managed envelope

https://github.com/getsentry/relay/blob/671bdf354b1fdc401d1a1c39430b7b1da3e70cec/relay-server/src/services/store.rs#L175

Which comes from the actual envelope store request start_time

https://github.com/getsentry/relay/blob/671bdf354b1fdc401d1a1c39430b7b1da3e70cec/relay-server/src/utils/managed_envelope.rs#L495-L500

schemas/ingest-monitors.v1.schema.json

@@ -38,7 +38,7 @@
           "$bytes": true
         },
         "start_time": {
-          "description": "The time relay produced the check-in to kafka. In seconds with floating precision.",
+          "description": "The time relay received the envelope containing the check-in. In seconds.",
           "type": "number"
         },
         "project_id": {