ci: add explicit least permissions to workflows

[shreemaan-abhishek]

Description

This PR explicitly defines the permissions block for several GitHub Actions workflows that were previously relying on the default (potentially permissive) settings.

By setting permissions: contents: read (and pull-requests: read where necessary), we ensure that the GITHUB_TOKEN used in these workflows follows the principle of least privilege. this acts as a security measure to limit the impact of a compromised workflow step.

With a compromised github token, a bad actor can:

• Push malicious code directly to the master branch.
• Create or delete releases and tags.
• Modify repository settings or secrets (if applicable).

Checklist

• I have explained the need for this PR and the problem it solves
• I have explained the changes or the new features added to this PR
• I have added tests corresponding to this change
• I have updated the documentation to reflect this change
• I have verified that this change is backward compatible (If not, please discuss on the APISIX mailing list first)