FINERACT-2544: update merge strategy guidance for FSIP-7

[mansi75]

Description

This PR removes the obsolete GitHub Action that enforced a one-commit-per-user rule on pull requests and updates the merge strategy guidance in CONTRIBUTING.md to align with the current Apache Fineract policy under FSIP-7.

Previously, the repository expected most PRs to consist of a single commit and included an automated check to fail PRs when the same contributor had multiple commits. However, the community has now adopted the traditional merge strategy in order to preserve commit history and retain verified signatures on individual commits. Because of that, the one-commit-per-user workflow is no longer valid and may incorrectly block legitimate contributions.

This change updates both repository behavior and contributor guidance so they are consistent with the new merge policy.

Checklist

• Removed the obsolete one-commit-per-user GitHub Action workflow
• Updated CONTRIBUTING.md to reflect the traditional merge strategy
• Clarified that multiple commits per PR are allowed
• Aligned repository checks and documentation with FSIP-7

[edk12564]

Hi @mansi75, please check the comment I made in https://issues.apache.org/jira/browse/FINERACT-2544?page=com.atlassian.jira.plugin.system.issuetabpanels%3Aall-tabpanel.

In short, I had an earlier PR that did the same thing. However, I think since you caught the README change as well, happy to go with your solution!

[mansi75]

@edk12564 Got it! Thank you :)