Fix backwards compatibility issue with Swift 5.7

[anferbui]

Bug/issue #, if applicable: fixes https://github.com/swiftlang/swift-docc-plugin/issues/94, rdar://134859979

Summary

swift-docc-plugin was not building with Swift 5.7 due to using newer syntax which did not exist yet.

Fixes the compilation issues by using older Swift syntax.

I've also updated our minimum supported version to Swift 5.7; older versions are no longer supported.

Dependencies

N/A

Testing

Steps:

1. Clone repository and set up docker image with Swift 5.7
2. Try to build swift-docc-plugin and it should fail without these changes, and succeed with them

docker run -v /local/path/to/swift-docc-plugin:/swift-docc-plugin -it swift:5.7-amazonlinux2
cd /swift-docc-plugin
swift build

Before:

/swift-docc-plugin/Plugins/Swift-DocC Convert/Symbolic Links/SwiftDocCPluginUtilities/CommandLineArguments/CommandLineArguments.swift:31:50: error: consecutive declarations on a line must be separated by ';'
    private var remainingOptionsOrFlags: [String].SubSequence
                                                 ^
[...]
/swift-docc-plugin/Plugins/Swift-DocC Convert/SwiftDocCConvert.swift:111:52: error: expected initial value after '='
            let dependencyArchivePaths: [String] = if isCombinedDocumentationEnabled {
                                                   ^
[...]
/swift-docc-plugin/Plugins/Swift-DocC Convert/Symbolic Links/SwiftDocCPluginUtilities/HelpInformation.swift:139:27: error: expected initial value after '='
        var description = if flagListText.count < 23 {
                          ^

After:

[85/85] Linking snippet-extract
Build complete! (4.95s)

Checklist

Make sure you check off the following items. If they cannot be completed, provide a reason.

• [ ] Added tests Not needed
• [x] Ran the ./bin/test script and it succeeded
• [ ] Updated documentation if necessary Not needed

[sofiaromorales]

@swift-ci please test

[anferbui]

@swift-ci please test