Support allow_duplicates on append processor

[stevejgordon]

Related to elastic/elasticsearch#61916
Contributes to #5198

[Mpdreamz]

I think its okay to simplify the Append cases to only run on 7.11.0 or include a AppendWithAllowDuplicates additional test class.

Easier to track then dynamically built json.

Also failing on the following CodeStandards tests:

Tests.CodeStandards.Descriptors.DescriptorMethodsTakingSingleValueTypeShouldBeNullable [FAIL]
  Failed Tests.CodeStandards.Descriptors.DescriptorMethodsTakingSingleValueTypeShouldBeNullable [166 ms]
  Error Message:
   Expected breakingDescriptors to be empty, but found {"allowDuplicates on method AllowDuplicates of Nest.AppendProcessorDescriptor`1 is not nullable"}.
  Stack Trace:

Descriptors taking value types should be nullable so that they be effectively reset.

[stevejgordon]

@Mpdreamz Ready for a re-review please

[Mpdreamz]

LGTM !

[github-actions]

The backport to master failed:

The process '/usr/bin/git' failed with exit code 1

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add .worktrees/backport-master master
# Navigate to the new working tree
cd .worktrees/backport-master
# Create a new branch
git switch --create backport-5208-to-master
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick ---mainline 1 2b75c5ef1623197dc67a96d37696fcf9007438c8
# Push it to GitHub
git push --set-upstream origin backport-5208-to-master
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-master

Then, create a pull request where the base branch is master and the compare/head branch is backport-5208-to-master.