Rename Get Channels API REST endpoint

[qreshi]

Description

• Updates references to common-util objects that were changed in Rename references for Get Channels API for Notifications common-utils#140
• Updates Get Channels REST endpoint from _plugins/_notifications/features/channels to _plugins/_notifications/channels

Issues Resolved

[List any issues this PR will resolve]

Check List

• New functionality includes testing.
  • All tests pass, including unit test, integration test and doctest
• New functionality has been documented.
  • New functionality has javadoc added
  • New functionality has user manual doc added
• Commits are signed per the DCO using --signoff

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.
For more information on following Developer Certificate of Origin and signing off your commits, please check here.

[qreshi]

Local build passed (also ran a local cluster for some sanity checks):

qreshi@186590ce20fd notifications % ./gradlew build
=======================================
OpenSearch Build Hamster says Hello!
  Gradle Version        : 7.3
  OS Info               : Mac OS X 10.16 (x86_64)
  JDK Version           : 14 (OpenJDK)
  JAVA_HOME             : /Library/Java/JavaVirtualMachines/jdk-14.0.1.jdk/Contents/Home
  Random Testing Seed   : 46791DBCE668ECB8
  In FIPS 140 mode      : false
=======================================

...

BUILD SUCCESSFUL in 5m 45s
55 actionable tasks: 23 executed, 32 up-to-date

Pasting the local build output since the GitHub Actions will fail since the common-utils changes are in PR alongside this one.