Fix: use eth_getGasPrice on xDai and Arbitrum

[katspaugh]

What it solves

Partially resolves #2837
Resolves #2999

How this PR fixes it

Instead of using a fixed gas price on xDai, request it from the blockchain.

I've done the same for Arbitrum, as it's recommended in their docs.

How to test it

• Create a tx on xDai
• Check the gasPrice in the advanced settings

Same on Arbitrum.

Screenshot

[github-actions]

CLA Assistant Lite All Contributors have signed the CLA.

[katspaugh]

Also removed this from the Rinkeby config. Etherscan should be more than enough.

[github-actions]

ESLint Summary View Full Report

Annotations are provided inline on the Files Changed tab. You can also see all annotations that were generated on the annotations page.

Type	Occurrences	Fixable
Errors	0	0
Warnings	0	0
Ignored	3	N/A

• Result: ✅ success
• Annotations: 0 total

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^{Report generated by eslint-plus-action}

[github-actions]

• Safe Rinkeby
• Safe Mainnet
• Safe Polygon
• Safe BSC
• Safe Arbitrum
• Safe xDai

[github-actions]

E2E Tests Failed
Check the results here: https://github.com/gnosis/safe-react-e2e-tests/actions/runs/1463167013

Failed tests:

• ❌ Safe Balances Safe Balances

[coveralls]

Pull Request Test Coverage Report for Build 1463142245

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 31.68%

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Totals
Change from base Build 1462240983:	0.0%
Covered Lines:	2985
Relevant Lines:	8383

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💛 - Coveralls

[francovenica]

Safe I used:
https://pr2997--safereact.review-safe.gnosisdev.com/app/xdai:0xB8d760a90a5ed54D3c2b3EFC231277e99188642A/transactions/queue

The tx still is not estimating enough for a tx. This is a "Send funds" tx with 1 owner policy

and of course it ends up failing

Should we bump up by a % every tx?

[francovenica]

Update:
In the previous comment the tx failed because of the gas limit, that is a known issue in xdai and is not part of this ticket.
I was told that the ticket is exclusive for the gasPrice.
So the gas price is not fixed in "1" anymore, is actually being estimated.

Looks good to me