Update Common.robot and requirements for correct LT variable handling

[shivanks-hue]

Pull Request: Update common.robot for LambdaTest iOS Execution
Summary
This update cleans up and simplifies the common.robot file used for Appium iOS tests.

Key Changes:

1. Replaced undefined ${version} with ${platformVersion}.
2. Updated LambdaTest Grid URL to use ${username} and ${accesskey} directly.
3. Removed unnecessary environment-based TRY/EXCEPT logic.
4. Cleaned up capability definitions and added consistent variables (e.g., devicelog=True).
5. Simplified Remote URL and APP ID handling.
6. General formatting and readability improvements.

[maheshr-lambdatest]

1. Uncomment the desired code lines in the files.
2. Modify the code to retrieve the username and access key from environment variables.
3. Review and update the README file to include initial setup steps for installation and instructions on how to run the script

[shivanks-hue]

I have uncommented the desired code lines in the files.
Modify the code to retrieve the username and access key from environment variables.
Update the README file to include initial setup steps for installation and instructions on how to run the tests.