Bump client Go module to 1.26 and update dependencies

[gregnr]

The client module still targets Go 1.17 with dependencies pinned to Nov 2021. The Dockerfile already builds with Go 1.26, so the go.mod is just lagging behind. Bumping it would let us lint the client with golangci-lint v2 in CI (currently skipped due to incompatibility) and pick up 4 years of dependency updates.

• Update client/go.mod from go 1.17 to go 1.26
• Run go mod tidy
• Add golangci-lint step back to build-client CI job
• Fix any lint findings