chore(ci): update post release workflow

[dreamorosi]

Description of your changes

This PR aims at fixing the issue described in the linked PR that caused the workflow in charge of leaving a comment & labelling issues related to a release to fail the operation.

Aside from formatting, the changes involve making sure that when the script is running (upon new release) it's considering only closed issues that were labeled as status/pending-release. The other main change, that should fix the issue, however involves fixing a bug around how the existing labels were filtered and merged with the new ones.

Related issues, RFCs

Issue number: #1493

Checklist

• My changes meet the tenets criteria
• I have performed a self-review of my own code
• I have commented my code where necessary, particularly in areas that should be flagged with a TODO, or hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my change is effective and works
• The PR title follows the conventional commit semantics

Breaking change checklist

Is it a breaking change?: NO

• I have documented the migration process
• I have added, implemented necessary warnings (if it can live side by side)

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