Add txId in logback.xml

[t-bast]

We changed the logging MDC key for tx-publishing in #2131, but forgot to add a formatting rule in logback.xml for that new field, so it wasn't written to logs.

[sstone]

LGTM but I find the PR description (and the choice of tag t: in logback.xml) a bit surprising as we're not using a bitcoin transaction id but rather a "publishing attempt id" which is a random id that is specific to eclair ?

[t-bast]

What other letter would you use that would make more sense?
i (which could have been id) and p (which could have been publish) are already used...
We can change from t to something else, but I couldn't find a letter that made more sense (and we don't care that much to be honest, we're more interested in the value it carries).

[sstone]

Fair enough.