cli -netinfo: various quick updates and fixes

Summary:

• update change "uptime" column name to "age" per suggestion by 0xB10C (Timo)

• add an additional digit to mping field width

• change m_networks_size from size_t to uint8_t, as size_t was a holdover from m_networks_size being defined as size_t m_networks.size() in a draft

• order Peer struct members by decreasing memory size

This is a backport of core#20115 [1/4]
https://github.com/bitcoin/bitcoin/pull/20115/commits/f8a1c4d9469cb496fdafaf6f4d94977687df9190

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/D10707