Remove maintain permissions

[GrantBirki]

There is no maintain permission on GitHub. Removing it from code base because it can bring confusion with maintain role.

Official documentation: https://docs.github.com/en/enterprise-cloud@latest/rest/collaborators/collaborators?apiVersion=2022-11-28#get-repository-permissions-for-a-user

Checks the repository permission of a collaborator. The possible repository permissions are admin, write, read, and none. Note: The permission attribute provides the legacy base roles of admin, write, read, and none, where the maintain role is mapped to write and the triage role is mapped to read. To determine the role assigned to the collaborator, see the role_name attribute, which will provide the full role name, including custom roles. The permissions hash can also be used to determine which base level of access the collaborator has to the repository. 

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related: github/branch-deploy#351

[Copilot]

PR Overview

This PR removes the unsupported "maintain" permission from the codebase for clarity and to align with GitHub's official permissions.

• Updated documentation in README.md to remove "maintain" permission references
• Adjusted unit tests in tests/functions/prechecks.test.js and valid-permissions.test.js to reflect the change
• Modified action.yml default and description accordingly

Reviewed Changes

File	Description
README.md	Updated examples and parameter definitions to remove the "maintain" permission
tests/functions/prechecks.test.js	Revised environment variable input for permissions to align with allowed permissions
tests/functions/valid-permissions.test.js	Updated test case and error message to remove references to "maintain"
action.yml	Changed default and documentation string to exclude "maintain"

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

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