Add inject_policy as default pipeline execution config strategy

What does this MR do and why?

Add inject_policy as default pipeline execution config strategy.

With the new inject_policy strategy, custom policy stages are no longer ignored when injected into the main pipeline.

We make it as default strategy, while supporting inject_ci for backward compatibility.

References

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• Backend MR: Inject custom pipeline execution policy stages (!178199 - merged)

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Existing inject_ci policy	New policy	Existing inject_policy policy

How to set up and validate locally

1. Create a project
2. Create a new pipeline execution policy
3. Verify that inject_policy is selected as a default strategy
4. Switch to .yaml mode and overwrite the strategy to inject_ci
5. Verify that the third, deprecated strategy shows up and the link to the docs is clickable

Related to #475152 (closed)