Update base fee in begin blocker

[yihuang]

Closes: #820

Description

Move base fee update from end blocker to begin blocker, to keep it compliant with go-ethereum.

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For contributor use:

• 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.
• Code follows the module structure standards.
• Wrote unit and integration tests
• Updated relevant documentation (docs/) or specification (x/<module>/spec/)
• Added relevant godoc comments.
• Added a relevant changelog entry to the Unreleased section in CHANGELOG.md
• Re-reviewed Files changed in the Github PR explorer

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For admin use:

• Added appropriate labels to PR (ex. WIP, R4R, docs, etc)
• Reviewers assigned
• Squashed all commits, uses message "Merge pull request #XYZ: [title]" (coding standards)

[codecov]

Codecov Report

Merging #822 (eb6f510) into main (423944b) will decrease coverage by 0.26%.
The diff coverage is 66.66%.

❗ Current head eb6f510 differs from pull request most recent head b813245. Consider uploading reports for the commit b813245 to get more accurate results

@@            Coverage Diff             @@
##             main     #822      +/-   ##
==========================================
- Coverage   57.31%   57.05%   -0.27%     
==========================================
  Files          72       72              
  Lines        5984     5985       +1     
==========================================
- Hits         3430     3415      -15     
- Misses       2357     2373      +16     
  Partials      197      197              

Impacted Files	Coverage Δ
x/feemarket/keeper/eip1559.go	94.54% <ø> (ø)
x/feemarket/keeper/abci.go	46.66% <50.00%> (-53.34%)	⬇️
app/app.go	83.84% <100.00%> (+0.04%)	⬆️

[fedekunze]

Ack, can you add a changelog entry?

[yihuang]

Ack, can you add a changelog entry?

done