[lint] disable opening_brace for multiline readability

* This rule `opening_brace` declares opening braces should be on the same line as the declaration
* However for multiline if / let statements, it makes it harder to read:

if let blah = blah,
   let foo = blah.foo {
    // do something, less readable
}

if let blah = blah,
   let foo = blah.foo
{
   // do something, more readable
}

* See linked PR for an upcoming option, so we can re-enable this rule

Author: nan-li

.swiftlint.yml

@@ -1 +1,4 @@
-line_length: 150
+line_length: 150
+disabled_rules:
+  # Track https://github.com/realm/SwiftLint/pull/5521
+  - opening_brace