chore(slack-lite): switch to pull_request_target so have access to repo secrets (#208)

No need for us to be running code from the PR itself, so this event is fine.
Before this change, a workflow run for a PR from a *fork* would have
'Secret source: None' and hence not have access to the SLACK_BOT_TOKEN

Also be clearer on the 'slack-lite' brand.

Refs: elastic/apm-agent-nodejs#4501

Author: trentm

.github/workflows/slack.yml renamed to .github/workflows/slack-lite.yml

@@ -11,19 +11,19 @@
 # 3. Optionally tweak the `if:` and `on:` sections below to control which issue
 #    and PR events are skipped.
 
-name: slack
+name: slack-lite
 
 env:
   SLACK_CHANNEL: "#apm-agent-node"
 
 on:
   issues:
     types: [opened, reopened, closed]
-  pull_request:
+  pull_request_target:
     types: [opened, ready_for_review, reopened, closed]
 
 jobs:
-  slack:
+  slack-lite:
     # Skip notification if:
     # - dependabot or renovate PRs, too noisy
     # - draft PRs