Remove SyntaxKind conformance to CaseIterable

There’s no good reason to iterate over all cases in `SyntaxKind` and if `SyntaxKind` conforms to `CaseIterable`, we can’t mark members of it as deprecated (https://github.com/apple/swift-syntax/pull/2237/files#r1359917461)

Author: ahoppen

CodeGeneration/Sources/generate-swift-syntax/templates/swiftsyntax/SyntaxKindFile.swift

@@ -19,7 +19,7 @@ let syntaxKindFile = SourceFileSyntax(leadingTrivia: copyrightHeader) {
   try! EnumDeclSyntax(
     """
     /// Enumerates the known kinds of Syntax represented in the Syntax tree.
-    public enum SyntaxKind: CaseIterable
+    public enum SyntaxKind
     """
   ) {
     DeclSyntax("case token")

Release Notes/511.md

@@ -16,6 +16,9 @@
 - Effect specifiers:
   - Description: The `unexpectedAfterThrowsSpecifier` node of the various effect specifiers has been removed.
   - Pull request: https://github.com/apple/swift-syntax/pull/2219
+- `SyntaxKind` removed conformance to `CaseIterable`
+  - Description: `SyntaxKind` no longer conforms to `CaseIterable` since there is no good use case to iterate over all syntax kinds. 
+  - Pull request: 
 
 
 ## Template

Sources/SwiftSyntax/generated/SyntaxKind.swift

@@ -13,7 +13,7 @@
 //===----------------------------------------------------------------------===//
 
 /// Enumerates the known kinds of Syntax represented in the Syntax tree.
-public enum SyntaxKind: CaseIterable {
+public enum SyntaxKind {
   case token
   case accessorBlock
   case accessorDeclList