fix: PR message not found

[gabrielmbmb]

Description

This PR updates the code of the slack-post-credentials actions to retrieve 500 messages in order to ensure that the PR opened message is found in the Slack channel.

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested

N/A

Checklist

• I followed the style guidelines of this project
• I did a self-review of my code
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• I filled out the contributor form (see text above)
• I have added relevant notes to the CHANGELOG.md file (See https://keepachangelog.com/)