Avoid using vendored lock in BroadcastAsyncSequence

Motivation:

ManagedBuffer is explicitly not 'Sendable' on Swift nightly builds. As a
result, our Lock type stops being 'Sendable'. This is only used by the
'BroadcastAsyncSequence'.

Modifications:

- Modify 'BroadcastAsyncSequence' to use 'Mutex'. One downside to this
  approach is that when a subscribe wants to consume an element and
  there's no element available, the lock must be released and then
  re-acquired once a continuation has been created.
- Remove our lock, and update NOTICES.txt to reflect the fact we no
  longer use a copy of NIOs lock.

Result:

- Less code
- CI passes

Author: glbrntt

NOTICES.txt

@@ -17,20 +17,6 @@ under the License.
 
 -------------------------------------------------------------------------------
 
-This product uses scripts derived from SwiftNIO's integration testing
-framework: 'test_01_allocation_counts.sh', 'run-nio-alloc-counter-tests.sh' and
-'test_functions.sh'.
-
-It also uses derivations of SwiftNIO's lock 'NIOLock.swift' and locked value box
-'NIOLockedValueBox.swift'.
-
-  * LICENSE (Apache License 2.0):
-    * https://github.com/apple/swift-nio/blob/main/LICENSE.txt
-  * HOMEPAGE:
-    * https://github.com/apple/swift-nio
-
----
-
 This product uses derivations of SwiftNIOHTTP2's implementation of case
 insensitive comparison of strings, found in 'HPACKHeader.swift'.