Bump wal-g to v3

[gabrielmocan]

Hello everyone,

After a lot of "popular pressure", wal-g team just cutted a brand new release

https://github.com/wal-g/wal-g/releases/tag/v3.0.0

Among other things, this release has two very anticipated features that will benefit Spilo project directly:

• Storage failover: this feature enables a secondary backup storage as failover in case the primary goes down. This is extremely useful as will prevent WAL directory filling up in case of storage outage (this will require a few more ENVs to configure the secondary storage);
• zstd returnal: this compression method has practically the same efficiency as brotli while being significantly less CPU intensive.

Hopefully we can see this bump as soon as possible to benefit from these very anticipated features.