Fix default value for instance level oid_batch_size

[AlexandreYang]

What does this PR do?

Fix default value for instance level oid_batch_size

Motivation

Instance level oid_batch_size is only available for corecheck SNMP integration and the default for oid_batch_size for corecheck SNMP integration is 5: https://github.com/DataDog/datadog-agent/blob/main/pkg/collector/corechecks/snmp/checkconfig/config.go#L30-L33

Additional Notes

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• PR title must be written as a CHANGELOG entry (see why)
• Files changes must correspond to the primary purpose of the PR as described in the title (small unrelated changes should have their own PR)
• PR must have changelog/ and integration/ labels attached

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[codecov]

Codecov Report

Merging #11018 (adbf96b) into master (8a0ec3f) will increase coverage by 0.00%.
The diff coverage is n/a.

Flag	Coverage Δ
snmp	88.09% <ø> (+0.04%)	⬆️

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[AlexandreYang]

/azp run PR Changes

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