## Issue Addressed
Fixes#2141
Remove [tempdir](https://docs.rs/tempdir/0.3.7/tempdir/) in favor of [tempfile](https://docs.rs/tempfile/3.1.0/tempfile/).
## Proposed Changes
`tempfile` has a slightly different api that makes creating temp folders with a name prefix a chore (`tempdir::TempDir::new("toto")` => `tempfile::Builder::new().prefix("toto").tempdir()`).
So I removed temp folder name prefix where I deemed it not useful.
Otherwise, the functionality is the same.
## Issue Addressed
#1992 and #1987, and also to be considered a continuation of #1751
## Proposed Changes
many changed files but most are renaming to align the code with the semantics of `--network`
- remove the `--network` default value (in clap) and instead set it after checking the `network` and `testnet-dir` flags
- move `eth2_testnet_config` crate to `eth2_network_config`
- move `Eth2TestnetConfig` to `Eth2NetworkConfig`
- move `DEFAULT_HARDCODED_TESTNET` to `DEFAULT_HARDCODED_NETWORK`
- `beacon_node`s `get_eth2_testnet_config` loads the `DEFAULT_HARDCODED_NETWORK` if there is no network nor testnet provided
- `boot_node`s config loads the config same as the `beacon_node`, it was using the configuration only for preconfigured networks (That code is ~1year old so I asume it was not intended)
- removed a one year old comment stating we should try to emulate `https://github.com/eth2-clients/eth2-testnets/tree/master/nimbus/testnet1` it looks outdated (?)
- remove `lighthouse`s `load_testnet_config` in favor of `get_eth2_network_config` to centralize that logic (It had differences)
- some spelling
## Additional Info
Both the command of #1992 and the scripts of #1987 seem to work fine, same as `bn` and `vc`
