Naively I can think of two ways forward:
0) Somehow patch out this dependency from
golang-github-transparency-dev-tessera.
1) ITP a golang-github-cenkalti-backoff-v5 for the new API and use it.
2) ITP a golang-github-cenkalti-backoff-v4 as a copy of the current
packaging, update all reverse dependecies replacing
golang-github-cenkalti-backoff-dev with
golang-github-cenkalti-backoff-v4-dev, and then package v5 as
golang-github-cenkalti-backoff-dev and use that in
golang-github-transparency-dev-tessera.