fix(ci): remove pull_request trigger from release-drafter workflow

The pull_request trigger was causing the Release Drafter action to fail
with a GitHub API validation error:

  'Validation Failed: target_commitish = refs/pull/N/merge is invalid'

When triggered by a pull_request event, the action tries to set the
release's target_commitish to refs/pull/N/merge, which GitHub rejects
as an invalid ref for a release.

The pull_request trigger was only needed for autolabeler support, which
is not configured in this repo. Removing it fixes the recurring failures
and keeps the release draft updated only on push to main (the intended
behavior).

Fixes: workflow runs 23369773059 and 23369768998

Author: Ritchie

.github/workflows/release-drafter.yml

@@ -5,10 +5,6 @@ on:
     # branches to consider in the event; optional, defaults to all
     branches:
       - main
-  # pull_request event is required only for autolabeler
-  pull_request:
-    # Only following types are handled by the action, but one can default to all as well
-    types: [opened, reopened, synchronize]
   # pull_request_target event is required for autolabeler to support PRs from forks
   # pull_request_target:
   #   types: [opened, reopened, synchronize]