Add older eras deprecation notice.

[carbolymer]

Changelog

- description: |
    Add for eras <= Babbage a deprecation notice.
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[Jimbo4350]

LGTM but a couple of comments to consider

[Jimbo4350]

This is fine for now but I think we should demand the user specify the era. Otherwise we have to plan a release around this value which is cumbersome.

[carbolymer]

100%. Actually, this default value is used only in legacy hierarchy, so actually this change is wrong. We don't want to change a default there. I'll revert to Babbage.