Add AirVPN server Jishui for Spain (Madrid)

[GreatNewHope]

Summary

This PR adds the missing AirVPN server "Jishui" located in Madrid, Spain to the Gluetun servers.json list.

The Jishui server was missing, which prevented users from connecting to it by setting SERVER_NAMES=Jishui.
This PR adds the corresponding WireGuard and OpenVPN entries following the established structure used for other Spanish servers (Mekbuda, Taurus, Eridanus).

Each (vpn type + hostname) entry maps to one IP, maintaining consistency with existing servers.

---

Evidence (DNS resolution)

The following dig queries were used to verify the IP addresses assigned to the Spanish servers:

➜  ~ dig ANY es.all.vpn.airdns.org +short 
185.93.182.170
130.195.210.106
194.99.104.34
185.183.106.2
2001:ac8:23:13:ad52:937:4aa0:9315
2001:ac8:23:a9:4a21:419c:ab3:85dd
2001:ac8:35:11::3
2001:ac8:23:11:f237:6c93:5ca8:c220

➜  ~ dig ANY es2.all.vpn.airdns.org +short
185.183.106.4
185.93.182.172
194.99.104.36
130.195.210.107
2001:ac8:23:11:b9e7:e96:7762:c04b
2001:ac8:23:a9:e3ec:2305:d90f:3ae5
2001:ac8:35:11::4
2001:ac8:23:13:487a:8eb6:519d:687f

➜  ~ dig ANY es3.all.vpn.airdns.org +short
185.93.182.173
185.183.106.5
130.195.210.108
194.99.104.37
2001:ac8:23:a9:9b55:17d5:f842:bdfd
2001:ac8:35:11::5
2001:ac8:23:13:f3b9:3ff5:7cc3:2a0b
2001:ac8:23:11:9819:fd0:4752:7faf

➜  ~ dig ANY es4.all.vpn.airdns.org +short
130.195.210.109
194.99.104.38
185.183.106.6
185.93.182.174
2001:ac8:23:13:5018:fcb1:23f4:59d1
2001:ac8:35:11::6
2001:ac8:23:11:776a:3d38:b55e:8b6f
2001:ac8:23:a9:62c3:3191:b275:16dc

➜  ~ dig ANY jishui.airservers.org +short
130.195.210.106

---

Changes

• Added Jishui server entries for:
  • WireGuard (es.vpn.airdns.org and es.ipv6.vpn.airdns.org)
  • OpenVPN (es3.vpn.airdns.org, es3.ipv6.vpn.airdns.org, es4.vpn.airdns.org, es4.ipv6.vpn.airdns.org)
• Each hostname points to one unique IP address, just like existing Spanish servers.
• Followed alphabetical order and project formatting conventions.

---

Notes

• Jishui IPs were carefully selected to avoid duplication with existing servers (Taurus, Mekbuda, Eridanus).
• JSON formatting was validated with jq.