By Max Hunder and Simon Lewis KYIV/BAKHMUT, Ukraine (Reuters) – Ukrainian forces raised their national flag on a recaptured Black Sea island on Thursday in a defiant act against Moscow, but Kyiv lost a main international supporter after British Prime Minister Boris Johnson said he would step down. Russian forces also shelled potential conquests in eastern Ukraine ahead of an expected new offensive. Moscow did not conceal its delight at the political demise of Johnson, a leader whom it has long criticised for arming Kyiv so energetically. “The moral of the story is: do not seek to destroy Russi…