Nightly e2e tests against release branch SDK

[julienrbrt]

Summary

Run a nightly GitHub Workflow that runs the IBC e2e test suite against the release branch of the maintained version of the SDK.

Problem Definition

The IBC E2E test suite discovered two bugs in the SDK: cosmos/cosmos-sdk#12888 / cosmos/cosmos-sdk#13808 and cosmos/cosmos-sdk#13728.
It would be useful to run those tests nightly against the release branch of the SDK for catching these issues before they sneak into a release.

Release branches are supposed to have no breaking API, so I think it should be possible.
Then the result of these tests could be sent to a Slack channel to alert us, like we do for our simulations tests: https://github.com/cosmos/cosmos-sdk/blob/main/.github/workflows/sims-nightly.yml, https://github.com/cosmos/cosmos-sdk/blob/main/.github/workflows/sims-046.yml.

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