Update requires in LoggerThreadSafeLevel

The `concurrent` require was [added][1] previously because a
`Concurrent::Map` was added to hold all of the log levels by `Thread` id.
However, the `Map` was later [removed][2] by storing the log levels in the
`Thread` locals (and later in `IsolatedExecutionState`) instead. The new
implementation additionally removed the `cattr_accessor` (removing the
need to require the "attribute_accessors" core_ext) and also replaced
the [usage][3] of `Fiber` with `Thread` (so the require for `Fiber` is also no
longer necessary).

Since `Concurrent`, `Fiber`, and `cattr_accessor` are no longer used here, we
can remove the requires. Since `Logger` is used here (but was previously
required by `concurrent`), a require was added for it.

[1]: 629efb6
[2]: 2379bc5
[3]: 56ec504

activesupport/lib/active_support/logger_thread_safe_level.rb

@@ -1,9 +1,7 @@
 # frozen_string_literal: true
 
 require "active_support/concern"
-require "active_support/core_ext/module/attribute_accessors"
-require "concurrent"
-require "fiber"
+require "logger"
 
 module ActiveSupport
   module LoggerThreadSafeLevel # :nodoc: