[FEATURE REQUEST] Support for testnet4

[twofaktor]

Describe the new feature or enhancement

Considering that official support for testnet4 will likely be released with the imminent Bitcoin Core v28, this is a feature request to BTC RPC explorer as well.

Additional context

Bitcoin Core testnet4 support (included with v28): https://github.com/bitcoin/bitcoin/blob/master/doc/release-notes/release-notes-28.0.md#testnet4bip94-support
Fulcrum support PR merged: cculianu/Fulcrum#255
Electrs support PR merged: romanz/electrs#1104 (edit)
LND coming support: lightningnetwork/lnd#8966
Electrumx support PR merged: spesmilo/electrumx#273

[schildbach]

Bitcoin Core 28.0, including testnet4 support, has been released.

I think the current green "testnet color" icons can continue to be used for testnet4.

[schildbach]

electrs 0.10.7, including testnet4 support, has been released.