fix(dashmate): missing fallback fee on testnet with core v20

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Issue being fixed or feature implemented

Dashmate failed to start core v20 with error about missing fallbackffee on testnet

What was done?

Add fallbackffee on testnet

How Has This Been Tested?

Untested

Breaking Changes

None, this has always been the fallbackfee on testnet I think

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have added or updated relevant unit/integration/functional/e2e tests
• I have made corresponding changes to the documentation

For repository code-owners and collaborators only

• I have assigned this pull request to a milestone

[lklimek]

this should also apply to all networks, including local & devnet (not sure about production)

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@PastaPastaPasta @thephez should this value be set on mainnet as well?

[knst]

From slack to keep history.

this should also apply to all networks, including local & devnet (not sure about production)

No, these changes (backport of bitcoin/bitcoin#16524) do not affect production (mainnet); it was already 0 in the past for mainnet.

As release note says:

• -fallbackfee was 0 (disabled) by default for the main chain, but 20000 by default for the test chains. Now it is 0 by default for all chains. Testnet and regtest users will have to add fallbackfee=20000 to their configuration if they weren't setting it and they want it to keep working like before.

For dash default value were not 20.000 but 1000. So, you need to specify in config fallbackfee=0.00001 to keep same behavior as before for testnet/regnet/etc.

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Looks like CI is failing because it depends on older v0.24-dev branch. How should I get the updated code on this branch? I tried both git merge and git rebase and both showed hundreds of conflicts, which doesn't seem possible since I only edited three lines here.