Loosen migrate command version 2 matcher

Previously, Smithy 2.0 files with a version of `"2"` would be upgraded
incorrectly, ending up with duplicate and invalid version control statements,
e.g. upgrading the following:

```smithy
$version: "2"

// ...
```

would result in:

```smithy
$version: "2.0"

$version: "2"

// ...
```

This change loosens the version 2.0 matcher to match version control
statements without the decimal suffix `.0`.

smithy-cli/src/main/java/software/amazon/smithy/cli/commands/MigrateCommand.java

@@ -71,7 +71,7 @@ final class MigrateCommand implements Command {
 
     private static final Logger LOGGER = Logger.getLogger(MigrateCommand.class.getName());
     private static final Pattern VERSION_1 = Pattern.compile("(?m)^\\s*\\$\\s*version:\\s*\"1\\.0\"\\s*$");
-    private static final Pattern VERSION_2 = Pattern.compile("(?m)^\\s*\\$\\s*version:\\s*\"2\\.0\"\\s*$");
+    private static final Pattern VERSION_2 = Pattern.compile("(?m)^\\s*\\$\\s*version:\\s*\"2(\\.0)?\"\\s*$");
     private final String parentCommandName;
 
     MigrateCommand(String parentCommandName) {

smithy-cli/src/test/java/software/amazon/smithy/cli/commands/MigrateCommandTest.java

@@ -114,6 +114,25 @@ public void testUpgradeDirectoryWithJar() throws Exception {
         assertDirEqual(baseDir.getParent().resolve("v2"), tempDir);
     }
 
+    @ParameterizedTest(name = "{1}")
+    @MethodSource("noopSource")
+    public void testUpgradeV2Noop(Path noopTestFilePath, String name) {
+        Model model = Model.assembler().addImport(noopTestFilePath).assemble().unwrap();
+        String actual = new MigrateCommand("smithy").upgradeFile(model, noopTestFilePath);
+        String expected = IoUtils.readUtf8File(noopTestFilePath);
+
+        if (!actual.equals(expected)) {
+            Assertions.fail("Expected models to be equal:\n\nActual:\n\n" + actual + "\n\nExpected:\n\n" + expected);
+        }
+    }
+
+    public static Stream<Arguments> noopSource() throws Exception {
+        Path start = Paths.get(MigrateCommandTest.class.getResource("upgrade/no-op").toURI());
+        return Files.walk(start)
+                .filter(path -> Files.isRegularFile(path))
+                .map(path -> Arguments.of(path, path.getFileName().toString().replace(".v2.smithy", "")));
+    }
+
     private void assertDirEqual(Path actualDir, Path expectedDir) throws Exception {
         Set<Path> files = Files.walk(actualDir)
                 .filter(Files::isRegularFile)

smithy-cli/src/test/resources/software/amazon/smithy/cli/commands/upgrade/no-op/decimal-suffix.v2.smithy

@@ -0,0 +1 @@
+$version: "2.0"

smithy-cli/src/test/resources/software/amazon/smithy/cli/commands/upgrade/no-op/no-decimal-suffix.v2.smithy

@@ -0,0 +1 @@
+$version: "2"