Make willingness to contribute in pr template a dropdown (#1076)

## What changes are included in this PR?

Make willingness in pr template an dropdown rather a checkbox, since it
only allows one selection.

Author: liurenjie1024

.github/ISSUE_TEMPLATE/iceberg_bug_report.yml

@@ -51,12 +51,12 @@ body:
       label: Expected behavior
       placeholder: >
         A clear and concise description of what you expected to happen.
-  - type: checkboxes
+  - type: dropdown
     attributes:
       label: Willingness to contribute
       description: >
         The Apache Iceberg community encourages bug fix contributions. Would you or another member of your organization be willing to contribute a fix for this bug to the Apache Iceberg codebase?
       options:
-        - label: I can contribute a fix for this bug independently
-        - label: I would be willing to contribute a fix for this bug with guidance from the Iceberg community
-        - label: I cannot contribute a fix for this bug at this time
+        - I can contribute a fix for this bug independently
+        - I would be willing to contribute a fix for this bug with guidance from the Iceberg community
+        - I cannot contribute a fix for this bug at this time

.github/ISSUE_TEMPLATE/iceberg_feature.yml

@@ -33,12 +33,12 @@ body:
       label: Describe the solution you'd like
       placeholder: >
         A clear and concise description of what you want to happen.
-  - type: checkboxes
+  - type: dropdown
     attributes:
       label: Willingness to contribute
       description: >
         The Apache Iceberg community encourages feature contributions. Would you or another member of your organization be willing to contribute to this feature for the Apache Iceberg Rust codebase?
       options:
-        - label: I can contribute to this feature independently
-        - label: I would be willing to contribute to this feature with guidance from the Iceberg Rust community
-        - label: I cannot contribute to this feature at this time
+        - I can contribute to this feature independently
+        - I would be willing to contribute to this feature with guidance from the Iceberg Rust community
+        - I cannot contribute to this feature at this time