cli -netinfo: make age column variable-width

Summary:
It has a wide possible range and the new name ("age" instead of "uptime") is much shorter.

This is a backport of core#20115 [2/4]
https://github.com/bitcoin/bitcoin/pull/20115/commits/33e987452f869c279f2491499939e51e0af8364c

Depends on D10707

Test Plan: ninja && src/bitcoind --daemon && sleep 5 && src/bitcoin-cli -netinfo 4 && src/bitcoin-cli stop

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10708