Disable development mode (swiftlang#8139)

Disable development mode in the release 6.1 branch.

### Motivation:

We should never release Swift Package Manager in development mode. So
this disables dev mode and brings in the functionality of `999.0` over
being introduced specific swift tool version (primarily in tests)

### Modifications:

Modify `SwiftVersion` `isDevelopment` to `false` and update references
to swift tool version `999.0` to a specific version.

### Result:

`swift test`

Author: bkhouri

Sources/Basics/SwiftVersion.swift

@@ -59,7 +59,7 @@ extension SwiftVersion {
     /// The current version of the package manager.
     public static let current = SwiftVersion(
         version: (6, 1, 0),
-        isDevelopment: true,
+        isDevelopment: false,
         buildIdentifier: getBuildIdentifier()
     )
 }

Tests/WorkspaceTests/WorkspaceTests.swift

@@ -206,15 +206,15 @@ final class WorkspaceTests: XCTestCase {
             do {
                 let ws = try createWorkspace(
                     """
-                    // swift-tools-version:999.0
+                    // swift-tools-version:5.9.2
                     import PackageDescription
                     let package = Package(
                         name: "foo"
                     )
                     """
                 )
 
-                XCTAssertMatch(try ws.interpreterFlags(for: packageManifest), [.equal("-swift-version"), .equal("6")])
+                XCTAssertMatch(try ws.interpreterFlags(for: packageManifest), [.equal("-swift-version"), .equal("5")])
             }
 
             do {