cmd: deprecate --testnet, use named networks instead (#20852)

* cmd/utils: make goerli the default testnet

* cmd/geth: explicitly rename testnet to ropsten

* core: explicitly rename testnet to ropsten

* params: explicitly rename testnet to ropsten

* cmd: explicitly rename testnet to ropsten

* miner: explicitly rename testnet to ropsten

* mobile: allow for returning the goerli spec

* tests: explicitly rename testnet to ropsten

* docs: update readme to reflect changes to the default testnet

* mobile: allow for configuring goerli and rinkeby nodes

* cmd/geth: revert --testnet back to ropsten and mark as legacy

* cmd/util: mark --testnet flag as deprecated

* docs: update readme to properly reflect the 3 testnets

* cmd/utils: add an explicit deprecation warning on startup

* cmd/utils: swap goerli and ropsten in usage

* cmd/geth: swap goerli and ropsten in usage

* cmd/geth: if running a known preset, log it for convenience

* docs: improve readme on usage of ropsten's testnet datadir

* cmd/utils: check if legacy `testnet` datadir exists for ropsten

* cmd/geth: check for legacy testnet path in console command

* cmd/geth: use switch statement for complex conditions in main

* cmd/geth: move known preset log statement to the very top

* cmd/utils: create new ropsten configurations in the ropsten datadir

* cmd/utils: makedatadir should check for existing testnet dir

* cmd/geth: add legacy testnet flag to the copy db command

* cmd/geth: add legacy testnet flag to the inspect command