fix(mempool): use no-op mempool as default (backport #19970)

[mergify]

Description

Closes: #XXXX

According to my understanding of the code here, the default mempool should be a no-op mempool. This will result in a FIFO mempool, which I believe is the expected default.

However, with current logic, the default max txs is picked up from the default mempool config which is > 0 (is 5000). As a result, the "defaultMempool" ends up being a SenderNonceMempool.

Additionally, this is conflicting with the default value of the corresponding flag

This PR introduces the changes to unify these two default parameters and keep the defaultMempool as a NoOpMempool

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This is an automatic backport of pull request #19970 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 1d2a795 has failed:

On branch mergify/bp/release/v0.50.x/pr-19970
Your branch is up to date with 'origin/release/v0.50.x'.

You are currently cherry-picking commit 1d2a795ea.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   baseapp/abci_test.go
	modified:   server/config/config.go
	modified:   types/mempool/mempool_test.go
	modified:   types/mempool/sender_nonce.go
	modified:   types/mempool/sender_nonce_test.go

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   CHANGELOG.md

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[tac0turtle]

this is api breaking is this needed?

[julienrbrt]

Not API break but indeed a behavior change, however current chains have already their app.toml written on disk, so they won't notice the behavior change and new chains will get immediately the expected behavior.