Remove PUT from the available item operations for the custom_objects endpoint

[dongilbert]

Q	A
Bug fix? (use the a.b branch)	[x]
New feature/enhancement? (use the a.x branch)	[ ]
Deprecations?	[ ]
BC breaks? (use the c.x branch)	[x]
Automated tests included?	[ ]
Related user documentation PR URL	mautic/mautic-documentation#...
Related developer documentation PR URL	mautic/developer-documentation#...
Issue(s) addressed	Fixes #...

Description:

Performing a PUT operation on the custom_objects/{id} endpoint has unexpected results and instead behaves like the PATCH endpoint. After discussion with the team, we decided to remove the PUT operation from the custom_objects endpoint.

Steps to test this PR:

1. Open this PR for testing locally (see docs on testing PRs here)
2. Log in to the instance, and navigate to instance_url.com/api/v2.
3. Scroll down or search for custom_objects
4. See that PUT is not available on the custom_objects/{id} endpoint

[rohitp19]

LGTM