Issue #3123936 by JFortune: Update core version requirements for Drupal 9.

.idea/.gitignore

+# Default ignored files
+/workspace.xml

.idea/bg_img_field.iml

+<?xml version="1.0" encoding="UTF-8"?>
+<module type="WEB_MODULE" version="4">
+  <component name="NewModuleRootManager">
+    <content url="file://$MODULE_DIR$" />
+    <orderEntry type="inheritedJdk" />
+    <orderEntry type="sourceFolder" forTests="false" />
+  </component>
+</module>
\ No newline at end of file

.idea/misc.xml

+<?xml version="1.0" encoding="UTF-8"?>
+<project version="4">
+  <component name="JavaScriptSettings">
+    <option name="languageLevel" value="ES6" />
+  </component>
+</project>
\ No newline at end of file

.idea/modules.xml

+<?xml version="1.0" encoding="UTF-8"?>
+<project version="4">
+  <component name="ProjectModuleManager">
+    <modules>
+      <module fileurl="file://$PROJECT_DIR$/.idea/bg_img_field.iml" filepath="$PROJECT_DIR$/.idea/bg_img_field.iml" />
+    </modules>
+  </component>
+</project>
\ No newline at end of file

.idea/vcs.xml

+<?xml version="1.0" encoding="UTF-8"?>
+<project version="4">
+  <component name="VcsDirectoryMappings">
+    <mapping directory="$PROJECT_DIR$" vcs="Git" />
+  </component>
+</project>
\ No newline at end of file

bg_img_field.info.yml

 name: Background Image Field
 description: Allows you to add responsive background images based on selctors.
 core: 8.x
+core_version_requirement: ^8 || ^9
 type: module
 package: Field types
 
 dependencies:
   - drupal:responsive_image
-  - token:token
+  - token:token
\ No newline at end of file