rename nim-libp2p main branch from unstable to master

[kaiserd]

We decided to simplify the branch structure for nim-libp2p:

• rename unstable to master; this will be our new trunk branch
• switch to https://trunkbaseddevelopment.com/
• remove the old master branch which was lagging behind

Related PRs for Nimbus and Waku:

• https://github.com/status-im/nimbus-eth2/pull/6253
• https://github.com/waku-org/nwaku/pull/2648

[kaiserd]

(completed)