[CI] Use agent-v5 branch of integrations-core

[olivielpeau]

What does this PR do?

Switch to use the agent-v5 branch of integrations-core

Motivation

The code of Agent v5 is meant to work with the agent-v5 branch of integrations-core now.

Testing

As long as the CI passes on the PR we should be good

Additional Notes

Should've done this when we created the agent-v5 branches on the other repos, but I missed it at the time.

[olivielpeau]

(CI is still failing, I'll have to continue looking for the actual root cause)