Remove targetPlatformVersion check from FinalityFlow and update docs (#4955)

In Corda 4, FinalityFlow was updated to become an initiated flow, in order to ensure a node does not have to accept any signed transaction it receives without being able to check it first. The old behaviour of FinalityFlow was gated behind a targetPlatformVersion check, to prevent apps targeting V4 from using the old behaviour.

This is problematic for a few reasons. For an app wishing to be backwards compatible with a version running on V3, this forces the app to set targetPlatformVersion = 3, even if the app is thoroughly tested against V4. This goes against the purpose of the targetPlatformVersion. Another consequence is that an app remains pinned to targetPlatformVersion = 3 until it is sure that there are no other apps running at a lower version in the network, which would prevent newer versions of the app from taking advantage of features gated behind targetPlatformVersion checks. (Note that the restriction only prevents a new version of the app from initiating FinalityFlow with the old version - the old version is able to initiate a FinalityFlow and the new version will handle it, assuming the app has been written correctly.)

This fix removes the targetPlatformVersion check from FinalityFlow, and also provides a few documentation updates to clarify what level of testing would be expected to set a targetPlatformVersion.

core/src/test/kotlin/net/corda/core/flows/FinalityFlowTests.kt

@@ -57,19 +57,6 @@ class FinalityFlowTests : WithFinality {
                 willThrow<IllegalArgumentException>())
     }
 
-    @Test
-    fun `prevent use of the old API if the CorDapp target version is 4`() {
-        val bob = createBob()
-        val stx = aliceNode.issuesCashTo(bob)
-        val resultFuture = CordappResolver.withCordapp(targetPlatformVersion = 4) {
-            @Suppress("DEPRECATION")
-            aliceNode.startFlowAndRunNetwork(FinalityFlow(stx)).resultFuture
-        }
-        assertThatIllegalArgumentException().isThrownBy {
-            resultFuture.getOrThrow()
-        }.withMessageContaining("A flow session for each external participant to the transaction must be provided.")
-    }
-
     @Test
     fun `allow use of the old API if the CorDapp target version is 3`() {
         val oldBob = createBob(cordapps = listOf(tokenOldCordapp()))