fix: Block deletion of devtronapp(devtron-operator chart) in helm apps

[prakash100198]

Description

Block deletion of Devtron(devtron-operator chart) helm apps. Deletion of the default app comes with some unwanted behaviour so for default devtron app delete option must not be shown on the UI and also must not be deleted from the backend.

Fixes:-#2708

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

• Tested locally multiple times by creating external apps via CLI and then deleting it, for devtron app it's blocking the deletion which is expected from the code changes.
• UI changes tested by deploying code changes on demo cluster, workinh as expected.

Checklist:

• The title of the PR states what changed and the related issues number (used for the release note).
• Does this PR requires documentation updates?
• I've updated documentation as required by this PR.
• 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.

Does this PR introduce a user-facing change?