Release v3.0.2 (#2617)

- bump version
- update release notes
- update links to Postgres Slack
- simplify /sync health-check endpoint code
- update unit-tests to cover missing lines

.github/ISSUE_TEMPLATE/bug_report.yml

@@ -6,7 +6,7 @@ body:
 - type: markdown
   attributes:
     value: |
-      If you have a question please post it on channel [#patroni](https://postgresteam.slack.com/archives/C9XPYG92A) in the [PostgreSQL Slack](https://postgresteam.slack.com).
+      If you have a question please post it on channel [#patroni](https://postgresteam.slack.com/archives/C9XPYG92A) in the [PostgreSQL Slack](https://join.slack.com/t/postgresteam/shared_invite/zt-1qj14i9sj-E9WqIFlvcOiHsEk2yFEMjA).
       Before reporting a bug please make sure to **reproduce it with the latest Patroni version**!
       Please fill the form below and provide as much information as possible.
       Not doing so may result in your bug not being addressed in a timely manner.

.github/ISSUE_TEMPLATE/config.yml

@@ -1,5 +1,5 @@
 blank_issues_enabled: false
 contact_links:
 - name: Question
-  url: https://postgresteam.slack.com/archives/C9XPYG92A
-  about: Please ask question about Patroni on Slack
+  url: https://join.slack.com/t/postgresteam/shared_invite/zt-1qj14i9sj-E9WqIFlvcOiHsEk2yFEMjA
+  about: "Please ask questions on channel #patroni in the PostgreSQL Slack"