replace reference to Swift 5.3 and 5.3.1 with Swift Next

Some places, such as test cases and comments, must have hard-coded version numbers. For them, some of the version numbers are changed from assuming 5.3.1 to assuming 5.4 as the Swift version where pull request #2937 lands.

Many version numbers in the tests are prepended with "999" to make them work.

2 FIXME comments are added to the beginning of `ToolsVersionLoader.swift` and `ToolsVersionLoaderTests.swift` explaining what to look out for, and what has to be done, when replacing Swift Next with a concurrent version.

Author: WowbaggersLiquidLunch

CHANGELOG.md

@@ -1,15 +1,14 @@
 Note: This is in reverse chronological order, so newer entries are added to the top.
 
-
-Swift 5.3.1
+Swift Next
 -----------
 * [#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 > 5.3. For example, `//swift-tools-version:	5.3.1` and `//		 swift-tools-version: 5.4` 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 ≥ `Next`. For example, `//swift-tools-version:	Next` and `//		 swift-tools-version: Next` 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.