fix: simplify Gemfile to allow GitHub dependabot to parse this file properly

[DeepDiver1975]

Description

GitHub dependabot cannot analyse the Gemfile due to error below

Error message:

Dependabot only supports uninterpolated string arguments to eval_gemfile. Got `plugins_path`

Motivation and Context

Allow GitHub dependabot to manage dependencies

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• Added an issue with details about all relevant changes in the iOS documentation repository.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.
• Added changelog files for the fixed issues in folder changelog/unreleased