Rename ethprice API endpoint

[vbaranov]

Motivation

From Discord:

Hey frens, anyone know what this is showing?
https://blockscout.com/poa/xdai/api?module=stats&action=ethprice

Changelog

Rename ethprice API endpoint to coinprice in order to eliminate confusion

Checklist for your Pull Request (PR)

• I added an entry to CHANGELOG.md with this PR
• If I added new functionality, I added tests covering it.
• If I fixed a bug, I added a regression test to prevent the bug from silently reappearing again.
• I checked whether I should update the docs and did so by submitting a PR to https://github.com/blockscout/docs
• If I added/changed/removed ENV var, I submitted a PR to https://github.com/blockscout/docs to update the list of env vars at https://github.com/blockscout/docs/blob/master/for-developers/information-and-settings/env-variables.md and I updated the version to master in the Version column. Changes will be reflected in this table: https://docs.blockscout.com/for-developers/information-and-settings/env-variables.
• If I add new indices into DB, I checked, that they are not redundant with PGHero or other tools

[coveralls]

Pull Request Test Coverage Report for Build 5b5c4d490bc6617292d45fcda4119bc67c29eb19-PR-3719

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 7 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.6%) to 64.393%

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Files with Coverage Reduction	New Missed Lines	%
test/block_scout_web/features/pages/chain_page.ex	1	90.0%
lib/block_scout_web/checksum_address.ex	6	55.56%

Totals
Change from base Build 80cb87571ec12ee8cbd96ef9461ceec886ea6b90:	-0.6%
Covered Lines:	2096
Relevant Lines:	3255

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💛 - Coveralls