Add support for -Dbuild.version_qualifier and remove usage of mapping types

[qreshi]

Description

• Adds support for -Dbuild.version_qualifier in build.gradle
• Changes OpenSearch version being used from 2.0.0 to 2.0.0-alpha1
• Removes usage of mapping types

Issues Resolved

#379 #375

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

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[qreshi]

The backend GitHub Actions are failing because the common-utils snapshot is a couple of days old which had some changes for Notifications (I believe the infra team is looking into the blocker but it's not due to common-utils itself).

The frontend GitHub Actions are failing because changes in OpenSearch-Dashboards seems to have caused minor differences in the rendered output in the Jest tests. I checked the output and tested manually and the functionality seems to be unaffected so I'll submit a follow-up PR to update the snapshots.

[qreshi]

Backend tests are passing locally though (with common-utils published locally):

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   : 24D2FCE440F7256E
  In FIPS 140 mode      : false
=======================================

...

BUILD SUCCESSFUL in 6m 39s
55 actionable tasks: 13 executed, 42 up-to-date