1.8.21 is now the lowest geth supported

[LefterisJP]

Description

Set 1.8.21 to lowest supported geth version

This is to have the eth_getTransactionCount fix: ethereum/go-ethereum#2880

PR review check list

Quality check list that cannot be automatically verified.

• Safety
  • The changes respect the necessary conditions for safety (https://raiden-network-specification.readthedocs.io/en/latest/smart_contracts.html#protocol-values-constraints)
• Code quality
  • Error conditions are handled
  • Exceptions are propagated to the correct parent greenlet
  • Exceptions are correctly classified as recoverable or unrecoverable
• Compatibility
  • State changes are forward compatible
  • Transport messages are backwards and forward compatible
• Commits
  • Have good messages
  • Squashed unecessary commits
• If it's a bug fix:
  • Regression test for the bug
    • Properly covers the bug
    • If an integration test is used, it could not be written as a unit test
• Documentation
  • A new CLI flag was introduced, is there documentation that explains usage?
• Specs
  • If this is a protocol change, are the specs updated accordingly? If so, please link PR from the specs repo.
• Is it a user facing feature/bug fix?
  • Changelog entry has been added

[palango]

Looks good. I'd also state this in the changelog.

[codecov]

Codecov Report

Merging #4983 into develop will decrease coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           develop   #4983      +/-   ##
==========================================
- Coverage    80.62%   80.6%   -0.02%     
==========================================
  Files          120     120              
  Lines        14595   14595              
  Branches      2262    2262              
==========================================
- Hits         11767   11765       -2     
- Misses        2157    2158       +1     
- Partials       671     672       +1

Impacted Files	Coverage Δ
raiden/constants.py	100% <100%> (ø)	⬆️
raiden/network/transport/matrix/transport.py	80.03% <0%> (-0.92%)	⬇️
raiden/network/proxies/token_network.py	50.18% <0%> (-0.74%)	⬇️
raiden/api/python.py	68.33% <0%> (-0.53%)	⬇️
raiden/network/transport/matrix/client.py	69.8% <0%> (+1.29%)	⬆️
raiden/utils/echo_node.py	69.49% <0%> (+1.69%)	⬆️
raiden/tasks.py	70.07% <0%> (+2.18%)	⬆️
raiden/storage/serialization/types.py	64.17% <0%> (+4.47%)	⬆️

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[LefterisJP]

Looks good. I'd also state this in the changelog.

good point