Increase version to 2.340 and bump other dependants

[meiswjn]

This PR bumps the version of:

• Plugin Framework
• Jenkins Core
• BOM

It also pins the version of the Credentials plugin to the latest release as required for #68

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[meiswjn]

I had a look at the failing test cases. Bumping Caffeine fixed one issue, but now another remains. I will be gone for a few weeks and not sure if I can check in the meantime. As I am not that deep in frontend testing, I appreciate contributions!

[PierreBtz]

@meiswjn I took the liberty of fixing the compilation error and the test error.

[meiswjn]

Thanks! I was struggling with figuring out why the button was still disabled, apparently you fixed it by waiting for some JS. I am wondering: How did you find out about this? Did you just know it from experience or somehow debugged the frontend components? Would be happy to learn about this.