Lower SDK minimum Go version after transport decoupling

[zeevdr]

Description

After the transport interface (#160) decouples the SDK from gRPC, lower the minimum Go version for the core SDK modules to Go 1.22.

Currently the SDK requires Go 1.24 because gRPC v1.79.3 does. Once gRPC moves to an optional transport sub-module, the core SDK can target Go 1.22 — widely used in production and the common floor for popular Go libraries.

Why Go 1.22

• Go supports the two most recent releases (currently 1.25 + 1.26), but libraries typically target 3-4 back for broad adoption
• Go 1.22 is the practical floor — still widely deployed, and 1.21 gains nothing useful
• Only one Go 1.22+ feature in SDK production code: range-over-int in retry.go (rewrite to classic for loop)

Prerequisites

• SDK transport interface: decouple from gRPC #160 (transport interface) must be complete

Acceptance criteria

• Core SDK modules (configclient, adminclient, configwatcher) set go 1.22 in go.mod
• gRPC transport adapter in a separate sub-module with its own go.mod (can require Go 1.24+)
• Rewrite range-over-int in retry.go to classic for loop
• CI tests against Go 1.22, 1.25, 1.26
• Document supported Go versions in README