fix getting agent pool available versions api path

[xizha162]

Accidentally included agent pool in the path for getting available agent pool versions API. This API is expected to be called before agent pool creation. So should not include agent pool name in the path.
(The server side implementation is correct and no changed is needed. Only update the swagger to match the implementation.)

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[AutorestCI]

Automation for azure-sdk-for-python

The initial PR has been merged into your service PR:
Azure/azure-sdk-for-python#7367

[AutorestCI]

Automation for azure-sdk-for-go

The initial PR has been merged into your service PR:
Azure/azure-sdk-for-go#5833

[azuresdkci]

Can one of the admins verify this patch?

[yaohaizh]

@yangl900 please help review the change as code owner. Thanks.

[yangl900]

LGTM

[KrisBash]

Signing off for ARM. OK with the breaking change on the basis of this being a bug fix in swagger