Now that 5.10 is the baseline, remove some old bits/refs.

Author: thomasvl

CompileTests/InternalImportsByDefault/Package@swift-5.9.swift renamed to CompileTests/InternalImportsByDefault/Package@swift-5.10.swift

@@ -1,4 +1,4 @@
-// swift-tools-version: 5.9
+// swift-tools-version: 5.10
 
 // Package.swift
 //

CompileTests/InternalImportsByDefault/Package@swift-5.8.swift

@@ -6,8 +6,6 @@
 // See LICENSE.txt for license information:
 // https://github.com/apple/swift-protobuf/blob/main/LICENSE.txt
 
-// This test only makes sense for Swift 5.9+ because 5.8 doesn't support access
-// level on imports.
 private import Foundation
 
 struct InternalImportsByDefault {

CompileTests/InternalImportsByDefault/Sources/InternalImportsByDefault/main.swift

@@ -1,4 +1,4 @@
-// swift-tools-version: 5.8
+// swift-tools-version: 5.10
 
 import PackageDescription
 

CompileTests/MultiModule/Package@swift-5.8.swift renamed to CompileTests/MultiModule/Package@swift-5.10.swift

@@ -1,4 +1,4 @@
-// swift-tools-version: 5.9
+// swift-tools-version: 5.10
 
 import PackageDescription