[IIS] Remove beta warning from application_pool metricset

[muthu-mps]

• Bug

What does this PR do?

Relates to this issue #35061

How to test this PR locally

• Install IIS metricbeat module
• Enable application_pool metrics collection
• Verify there is no beta warnings.

Related issues

• Closes [Metricbeat IIS] Remove Beta warning flag from application_pool. #35061

[mergify]

This pull request does not have a backport label.
If this is a bug or security fix, could you label this PR @muthu-mps? 🙏.
For such, you'll need to label your PR with:

• The upcoming major version of the Elastic Stack
• The upcoming minor version of the Elastic Stack (if you're not pushing a breaking change)

To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-v8./d.0 is the label to automatically backport to the 8./d branch. /d is the digit

[elasticmachine]

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• Start Time: 2023-05-23T11:13:40.190+0000

• Duration: 61 min 52 sec

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Test	Results
Failed	0
Passed	1398
Skipped	90
Total	1488

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[shmsr]

The changes look good to me but the PR title seems a little off. Suggestion:

[IIS] Remove beta warning from application_pool metricset

^^ is similar to the title of the issue linked.

[ritalwar]

Entry missing in the changelog. Could you please add it?

[ritalwar]

LGTM!