rubocop: update for 0.30.0

v0.30.0 has killed a couple of the methods being used and is throwing
warnings around like candy. This switches to the new methods.

Rubocops are obviously a matter of choice, and I’ve added choice here
based on what was said in previous PRs and my own personal views on
readability. Happy to be told to change things to different variables.

For upstream detail, see:

*
https://github.com/bbatsov/rubocop/blob/master/relnotes/v0.30.0.md#chang
es
* https://github.com/bbatsov/rubocop/pull/1721/files
* https://github.com/bbatsov/rubocop/pull/1655/files

Closes Homebrew/homebrew#38655.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>

Library/.rubocop.yml

@@ -10,6 +10,14 @@ Style/StringLiterals:
 Style/StringLiteralsInInterpolation:
     EnforcedStyle: double_quotes
 
+# percent-x is allowed for multiline
+Style/CommandLiteral:
+    EnforcedStyle: mixed
+
+# paths abound, easy escape
+Style/RegexpLiteral:
+    EnforcedStyle: slashes
+
 # no metrics for formulas
 Metrics/AbcSize:
     Enabled: false
@@ -48,10 +56,6 @@ Style/EmptyLineBetweenDefs:
 Style/NumericLiterals:
     Enabled: false
 
-# paths abound, easy escape
-Style/RegexpLiteral:
-    MaxSlashes: 0
-
 # consistency and readability when faced with string interpolation
 Style/PercentLiteralDelimiters:
     PreferredDelimiters:
@@ -101,12 +105,6 @@ Style/WordArray:
 Style/UnneededCapitalW:
     Enabled: false
 
-# percent-x is allowed for multiline
-# TODO: enforce when rubocop has fixed this
-# https://github.com/bbatsov/rubocop/issues/1397
-Style/UnneededPercentX:
-    Enabled: false
-
 # TODO: rubocop bug regarding __END__
 # see https://github.com/bbatsov/rubocop/issues/1541
 Style/TrailingWhitespace: