[WIP] Address failing tests from ethereum/tests ``v11.1``

[fselmo]

What was wrong?

Related to #2075

• Difficulty calculation was taking into account the parent difficulty which would slip below the DIFFICULTY_MINIMUM value if the parent difficulty was less than the allowed minimum.

How was it fixed?

• Take the max only between the difficulty calculation and the actual minimum allowed value.

Todo:

• [x] Clean up commit history
• [x] Add entry to the release notes

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