CLI - Default config lists maincloud, not testnet

[bfops]

Description of Changes

Replaced testnet with maincloud in the default config file.

I did not replace it in any of our tests, since those test config files don't actually need to be synced with our real config files. I can replace there too if that's preferred.

API and ABI breaking changes

Not breaking.

Expected complexity level and risk

1

Testing

• Remove config file, run a CLI command, see that maincloud is in the config file now

$ rm ~/.config/spacetime/cli.toml

$ cargo run -p spacetimedb-cli -- server add foo --url https://foo.spacetimedb.com --no-fingerprint
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.24s
     Running `target/debug/spacetimedb-cli server add foo --url 'https://foo.spacetimedb.com' --no-fingerprint`
Saving config to /home/lead/.config/spacetime/cli.toml.
WARNING: This command is UNSTABLE and subject to breaking changes.

Host: foo.spacetimedb.com
Protocol: https
Saving config to /home/lead/.config/spacetime/cli.toml.

$ cat ~/.config/spacetime/cli.toml
default_server = "local"

[[server_configs]]
nickname = "local"
host = "127.0.0.1:3000"
protocol = "http"

[[server_configs]]
nickname = "maincloud"
host = "maincloud.spacetimedb.com"
protocol = "https"

[[server_configs]]
nickname = "foo"
host = "foo.spacetimedb.com"
protocol = "https"

[cloutiertyler]

This LGTM. We just want to merge at a strategic time so as to not mess up the BitCraft team as much as possible.