ci: fix update of plugin or theme fields

See [1] for the reference of value field (ProjectV2FieldValue type),
and [2] for sample application code.

[1] https://docs.github.com/en/graphql/reference/input-objects#projectv2fieldvalue
[2] https://docs.github.com/en/enterprise-cloud@latest/issues/planning-and-tracking-with-projects/automating-your-project/automating-projects-using-actions#example-workflow-authenticating-with-a-github-app

.github/workflows/project.yml

@@ -113,7 +113,9 @@ jobs:
                 projectId: $project
                 itemId: $item
                 fieldId: $plugin_field
-                value: $plugin_value
+                value: {
+                  text: $plugin_value
+                }
               }) {
                 projectV2Item {
                   id
@@ -123,7 +125,9 @@ jobs:
                 projectId: $project
                 itemId: $item
                 fieldId: $theme_field
-                value: $theme_value
+                value: {
+                  text: $theme_value
+                }
               }) {
                 projectV2Item {
                   id