Deprecate fix

[epark272]

ActionUpdateThread.OLD_EDT is being deprecated, and errors will pop up when running IntelliJ (even when the plugin is not actively used). Here is the error message:

com.intellij.diagnostic.PluginException: `ActionUpdateThread.OLD_EDT` is deprecated and going to be removed soon. 'edu.berkeley.cs61b.plugin.CheckStyleAction' must override `getActionUpdateThread()` and chose EDT or BGT. See ActionUpdateThread javadoc. [Plugin: edu.berkeley.cs61b.plugin]
	at com.intellij.diagnostic.PluginProblemReporterImpl.createPluginExceptionByClass(PluginProblemReporterImpl.java:23)
	at com.intellij.diagnostic.PluginException.createByClass(PluginException.java:90)
	at com.intellij.diagnostic.PluginException.reportDeprecatedUsage(PluginException.java:125)
	at com.intellij.openapi.actionSystem.ActionUpdateThreadAware.getActionUpdateThread(ActionUpdateThreadAware.java:21)
	at com.intellij.openapi.actionSystem.AnAction.getActionUpdateThread(AnAction.java:201)

Some points of concern:

• I don't know how to test the plugin locally, so I did not do any testing.
• ActionUpdateThread.BGT should be fine for our use cases?
• I was unable to find any documentation on ActionUpdateThread and which version it was being deprecated from. There is a small but unlikely (in my opinion) chance that this will break the plugin on older versions of IntelliJ.