open EIP2028 when Istanbul version enabled

[KamiD]

Closes :cosmos/ethermint#730

• Targeted PR against correct branch (see CONTRIBUTING.md)

• Linked to github-issue with discussion and accepted design OR link to spec that describes this work.

• Wrote tests

• Updated relevant documentation (docs/)

• Added a relevant changelog entry to the Unreleased section in CHANGELOG.md

• Reviewed Files changed in the github PR explorer

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For Admin Use:

• Added appropriate labels to PR (ex. wip, ready-for-review, docs)
• Reviewers Assigned
• Squashed all commits, uses message "Merge PR #XYZ: [title]" (coding standards)

[codecov-io]

Codecov Report

Merging #579 (d6b635c) into dev (4a06840) will decrease coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##              dev     #579      +/-   ##
==========================================
- Coverage   66.65%   66.62%   -0.03%     
==========================================
  Files         306      306              
  Lines       19009    19011       +2     
==========================================
- Hits        12670    12666       -4     
- Misses       5623     5629       +6     
  Partials      716      716              

Impacted Files	Coverage Δ
x/evm/types/chain_config.go	100.00% <100.00%> (ø)
x/evm/types/state_transition.go	93.16% <100.00%> (ø)
x/backend/keeper/keeper_channel.go	45.16% <0.00%> (-4.84%)	⬇️

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