chore: remove v8.3 and v8.4 from PR template (#21377)

Author: qiancai

.github/pull_request_template.md

@@ -26,8 +26,6 @@ For details, see [tips for choosing the affected versions](https://github.com/pi
 - [ ] master (the latest development version)
 - [ ] v9.0 (TiDB 9.0 versions)
 - [ ] v8.5 (TiDB 8.5 versions)
-- [ ] v8.4 (TiDB 8.4 versions)
-- [ ] v8.3 (TiDB 8.3 versions)
 - [ ] v8.1 (TiDB 8.1 versions)
 - [ ] v7.5 (TiDB 7.5 versions)
 - [ ] v7.1 (TiDB 7.1 versions)

README.md

@@ -29,8 +29,8 @@ Currently, we maintain the following versions of TiDB documentation in different
 | :---------|:----------|
 | [`master`](https://github.com/pingcap/docs/tree/master) | The latest development version |
 | [`release-8.5`](https://github.com/pingcap/docs/tree/release-8.5) | 8.5 LTS (Long-Term Support) |
-| [`release-8.4`](https://github.com/pingcap/docs/tree/release-8.4) | 8.4 Development Milestone Release |
-| [`release-8.3`](https://github.com/pingcap/docs/tree/release-8.3) | 8.3 Development Milestone Release |
+| [`release-8.4`](https://github.com/pingcap/docs/tree/release-8.4) | 8.4 Development Milestone Release (Archived documentation, no longer updated) |
+| [`release-8.3`](https://github.com/pingcap/docs/tree/release-8.3) | 8.3 Development Milestone Release (Archived documentation, no longer updated) |
 | [`release-8.2`](https://github.com/pingcap/docs/tree/release-8.2) | 8.2 Development Milestone Release (Archived documentation, no longer updated) |
 | [`release-8.1`](https://github.com/pingcap/docs/tree/release-8.1) | 8.1 LTS (Long-Term Support) |
 | [`release-8.0`](https://github.com/pingcap/docs/tree/release-8.0) | 8.0 Development Milestone Release (Archived documentation, no longer updated) |