refactor!: remove payment gateways (and related things)

[phot0n]

This pr aims to remove payment gateways from frappe and move it to a separate app.

TODO:

• resolve conflicts/update the branch to latest changes
• update the new repo to latest changes -> https://github.com/frappe/payments
• add integration request doctype back
• remove get_payment_gateway_controller safe exec (util has been shifted to payments app)
• fix erpnext stuff (+ add as a required_app) -> fix: misc payments fixes erpnext#31690
• patch(es)
• depreciation warning in v13 -> chore: add depreciation warning for payment gateways inlieu of payments app #17601
• manual testing - Tested Razorpay, worked fine

[stale]

This pull request has been automatically marked as stale because it has not had recent activity. It will be closed within 3 days if no further activity occurs, but it only takes a comment to keep a contribution alive :) Also, even if it is closed, you can always reopen the PR when you're ready. Thank you for contributing.

[stale]

This pull request has been automatically marked as stale because it has not had recent activity. It will be closed within 3 days if no further activity occurs, but it only takes a comment to keep a contribution alive :) Also, even if it is closed, you can always reopen the PR when you're ready. Thank you for contributing.

[stale]

This pull request has been automatically marked as stale because it has not had recent activity. It will be closed within 3 days if no further activity occurs, but it only takes a comment to keep a contribution alive :) Also, even if it is closed, you can always reopen the PR when you're ready. Thank you for contributing.

[surajshetty3416]

@phot0n please update migration guide. Migration guide should mention that payments related code was moved to a separate app with app link.