[JENKINS-72370][JENKINS-11889] fix SimpleScheduledRetentionStrategy with inbound agents

[mawinter69]

By implementing isAcceptingTasks the availability strategy ensures that no builds can start when the agent should not do anything.
The current behaviour with an inbound agent is that controller and agent play jojo. The strategy disconnects the agent, the agents java process detects the disconnect and reconnects to the controller, with the strategy again disconnecting the agent and so on.
So the change will only disconnect the agent when the controller can itself launch the agent, this means inbound agents are not disconnected, to avoid playing jojo. The agent will just not accept new tasks for execution.

The change also avoids the problem in [JENKINS-11889] for outbound agents where the accepting tasks of an agents seems to be not reset when changing the availability strategy to always on.

See JENKINS-72370.
See JENKINS-11889.

Testing done

Manual testing as described in the Jira ticket.

Proposed changelog entries

• Fix SimpleScheduledRetentionStrategy on inbound agents. Allow suspended agents to again accept tasks when they are reconnected and the configured scheduling policy is enabled.

Proposed upgrade guidelines

N/A

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[timja]

/label ready-for-merge

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This PR is now ready for merge, after ~24 hours, we will merge it if there's no negative feedback.

Thanks!