.Net Agents: Update experimental meta-data for graduation

[crickman]

Motivation and Context

As part of Agent Graduation, the Experimental attributes must be more precisely defined.

• Fixes: .Net Agents - Mark experimental features #10535
• Fixes: .Net Agent Graduation - Bump package from beta prior to graduation #10303

Description

Apply Experimental attributes for non-graduating artifacts for:

• Microsoft.SementicKernel.Agents.Abstractions
• Microsoft.SementicKernel.Agents.Core
• Microsoft.SementicKernel.Agents.OpenAI

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the SK Contribution Guidelines and the pre-submission formatting script raises no violations
• All unit tests pass, and I have added new tests where possible
• I didn't break anyone 😄

[markwallace-microsoft]

LGTM Need to get feedback from @RogerBarreto regarding the content graduation changes.

I changed the abstractions and core to have an rc suffix as agreed with LT.

[RogerBarreto]

We are planning in deprecating our KernelContens in favor or MEAI.AIContents in the mid/long term.

For graduation purposes suggest moving all agent specific Contents to Agent.Abstractions project graduating them there instead.