Rename v0.34.x-celestia branch to v1.x

[rootulp]

Context

This repo currently has a branch named v0.34.x-celestia. That name was motivated by the source code on that branch originating from upstream Tendermint / CometBFT 0.34.x.

Problem

v0.34.x-celestia doesn't convey the intended purpose of the branch. IMO that branch should contain code for the v1.x releases of this repo. E.g. https://github.com/celestiaorg/celestia-core/releases/tag/v1.35.0-tm-v0.34.29

Proposal

• Rename v0.34.x-celestia branch to v1.x
• Update the section in the README about supported branches
• Update any CI workflows and settings that explicitly reference v0.34.x-celestia