fix: update pg_version input to be optional and set default value to __NULL__

[arya-girish-k]

Description

Update pg_version input to be optional and set default value to __NULL__ in ibm_catalog.json.
Git_issue

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Release notes content

Update pg_version input to be optional and set default value to __NULL__ in ibm_catalog.json.

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[arya-girish-k]

/run pipeline

[shemau]

/run pipeline

[shemau]

LGTM

[terraform-ibm-modules-ops]

🎉 This PR is included in version 3.21.7 🎉

The release is available on:

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• v3.21.7

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