Maintain SourceKit-mode as long as custom rules don't support SwiftSyntax

[SimplyDanny]

#6129 is required for .swiftsyntax to become the default.

[SwiftLintBot]

	1 Warning
⚠️	If this is a user-facing change, please include a CHANGELOG entry to credit yourself! You can find it at CHANGELOG.md.

	18 Messages
📖	Building this branch resulted in the same binary size as when built on main.
📖	Linting Aerial with this PR took 0.81 s vs 0.82 s on main (1% faster).
📖	Linting Alamofire with this PR took 1.02 s vs 1.02 s on main (0% slower).
📖	Linting Brave with this PR took 6.94 s vs 6.94 s on main (0% slower).
📖	Linting DuckDuckGo with this PR took 20.34 s vs 20.42 s on main (0% faster).
📖	Linting Firefox with this PR took 10.47 s vs 10.49 s on main (0% faster).
📖	Linting Kickstarter with this PR took 7.78 s vs 7.78 s on main (0% slower).
📖	Linting Moya with this PR took 0.45 s vs 0.45 s on main (0% slower).
📖	Linting NetNewsWire with this PR took 2.39 s vs 2.39 s on main (0% slower).
📖	Linting Nimble with this PR took 0.63 s vs 0.64 s on main (1% faster).
📖	Linting PocketCasts with this PR took 7.14 s vs 7.06 s on main (1% slower).
📖	Linting Quick with this PR took 0.41 s vs 0.41 s on main (0% slower).
📖	Linting Realm with this PR took 3.46 s vs 3.48 s on main (0% faster).
📖	Linting Sourcery with this PR took 1.85 s vs 1.86 s on main (0% faster).
📖	Linting Swift with this PR took 4.28 s vs 4.3 s on main (0% faster).
📖	Linting VLC with this PR took 1.13 s vs 1.13 s on main (0% slower).
📖	Linting Wire with this PR took 17.07 s vs 17.04 s on main (0% slower).
📖	Linting WordPress with this PR took 10.02 s vs 10.02 s on main (0% slower).

Here's an example of your CHANGELOG entry:

* Maintain SourceKit-mode as long as custom rules don't support SwiftSyntax.  
  [SimplyDanny](https://github.com/SimplyDanny)
  [#issue_number](https://github.com/realm/SwiftLint/issues/issue_number)

note: There are two invisible spaces after the entry's text.

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