Improve bug report template.

- Add an input field for the Guava version.
- Add Java 21 to the platforms list.
  - The label doesn't exist yet, but we can create it when necessary.

Fixes #6839

RELNOTES=n/a
PiperOrigin-RevId: 652949305

.github/ISSUE_TEMPLATE/bug_report.yaml

@@ -6,7 +6,18 @@ body:
     attributes:
       value: >
         Thank you for filing a bug report. Please help us identify and resolve the bug by filling
-        out the following fields.
+        out the following fields. Before we begin, please make sure that the bug is still present in
+        the [latest](https://github.com/google/guava/releases/latest) version of Guava available.
+        If it's already fixed in the latest version of Guava, then please just update your Guava
+        version instead.
+
+  - type: input
+    attributes:
+      label: Guava Version
+      description: Which version of Guava are you using?
+      placeholder: e.g., 33.2.1-jre
+    validations:
+      required: true
 
   - type: textarea
     attributes:
@@ -72,6 +83,7 @@ body:
         - Java 8
         - Java 11
         - Java 17
+        - Java 21
 
   - type: checkboxes
     attributes:
@@ -81,3 +93,7 @@ body:
             I agree to follow the
             [code of conduct](https://github.com/google/.github/blob/master/CODE_OF_CONDUCT.md).
           required: true
+        - label: >
+            I can reproduce the bug with the
+            [latest](https://github.com/google/guava/releases/latest) version of Guava available.
+          required: true