Fixed & changed namespace validation

[milindk8]

Description

Fixed & made changes in the regex of namespace validation to allow users to create a namespace starting with an alphanumeric character in add environment in cluster & in cd pipeline

Fixes # ISSUE 497

Type of change

• [x] 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 not work as expected)
• [ ] This change requires a documentation update

Testing

Tested by creating an environment, the namespace in global configuration & in cd pipeline

Checklist:

• [x] The title of the PR states what changed and the related issues number (used for the release note).
• [ ] Does this PR require documentation updates?
• [ ] I've updated documentation as required by this PR.
• [x] I have performed a self-review of my own code
• [ ] I have commented my code, particularly in hard-to-understand areas
• [ ] I have tested it for all user roles
• [ ] I have added all the required unit/api test cases