Add a line warning deprecation of the CobaltProvider

[yadudoc]

Is your feature request related to a problem? Please describe.

The CobaltProvider was deprecated recently with #3101. It would be good to add a mention of that to the Execution Provider section https://parsl.readthedocs.io/en/stable/userguide/execution.html#execution-providers>

Describe the solution you'd like

Add a line to the text highlighted below that the CobaltProvider is deprecated and will be removed by 2024.04 like shown below:

Steps to solution

1. Add a line following the description of CobaltProvider in this file docs/userguide/execution.rst
2. Follow steps here https://github.com/Parsl/parsl/blob/master/docs/README.rst to build and check the docs.

[crocmons]

Hello, I want to work on this issue as an outreachy contributor. @yadudoc can you assign me this task?

[MunishMummadi]

I created a pull request #3145 . Please have a look

[christailu]

hello @yadudoc , can you take a look at #3171.
Thanks!

[SobiaAman]

Hello @benclifford i've done this but waiting for my first pull request to be viewed to make another pull request. Plus @benclifford is it okay that multiple contributors are doing the same thing and making the PR Or it should be done by only one contributor that comes first. Kindly clear that, i'm little bit confused in it. Thanks

[SobiaAman]

Hello @benclifford i have made PR on it 3194