[5.4] [PackageLoading] replace Next with 5.4 according to notes/instructions left previously

Also added `ToolsVersion.v5_4`.

Author: WowbaggersLiquidLunch

CHANGELOG.md

@@ -1,14 +1,14 @@
 Note: This is in reverse chronological order, so newer entries are added to the top.
 
-Swift Next
+Swift 5.4
 -----------
 * [#2937]
 
   * Improvements
 
     `Package` manifests can now have any combination of leading whitespace characters. This allows more flexibility in formatting the manifests.
 
-    [SR-13566] The Swift tools version specification in each manifest file now accepts any combination of _horizontal_ whitespace characters surrounding `swift-tools-version`, if and only if the specified version ≥ `Next`. For example, `//swift-tools-version:	Next` and `//		 swift-tools-version: Next` are valid.
+    [SR-13566] The Swift tools version specification in each manifest file now accepts any combination of _horizontal_ whitespace characters surrounding `swift-tools-version`, if and only if the specified version ≥ `5.4`. For example, `//swift-tools-version:	5.4` and `//		 swift-tools-version: 5.4` are valid.
 
     All [Unicode line terminators](https://www.unicode.org/reports/tr14/) are now recognised in `Package` manifests. This ensures correctness in parsing manifests that are edited and/or built on many non-Unix-like platforms that use ASCII or Unicode encodings.