Use the Go 1.27 toolchain for development and CI

[barnabasbusa]

Toolchain-only change: the go directive stays at 1.25.0, so the minimum Go version for dependent modules does not change.

• go.mod: toolchain go1.27.0 — only applies when go-eth2-client is the main module (local dev, CI), not to consumers
• workflows: setup-go on 1.27 for lint/revive/test
• custom golangci-lint: build v2.13.1 (supports Go 1.27) and include the Go version in the custom-gcl cache key, so a cached binary built with an older Go is not reused against a 1.27 target (the Go language version used to build golangci-lint is lower than the targeted Go version)
• Go 1.27 changed the encoding/json unmarshal error text (it now names the outermost struct and includes array indices in the field path, e.g. Go struct field inclusionListJSON.transactions → inclusionListJSON.transactions.0). Updated the 31 expected error strings in the affected *_test.go files so go test ./... passes on the toolchain the repo now builds with; no code changes.

go test ./... passes on go1.27.0. Ran the custom golangci-lint v2.13.1 built with 1.27 on master and this branch: same 79 pre-existing issues, no new ones.

https://claude.ai/code/session_01E47DJeAkHBdyG1G3Ld7fzK

[redpandabot]

Skipped automated review — PR is too large for a useful single-shot review.

• changed files: 24 (limit 80)
• diff bytes: 586217 (limit 500000)

---

_{Reviewed @ 7654625c
"Every system eventually becomes legacy."}

[redpandabot]

Skipped automated review — PR is too large for a useful single-shot review.

• changed files: 24 (limit 80)
• diff bytes: 586187 (limit 500000)

---

_{Reviewed @ 72579c0d
"Every system eventually becomes legacy."}

[redpandabot]

Skipped automated review — PR is too large for a useful single-shot review.

• changed files: 24 (limit 80)
• diff bytes: 586195 (limit 500000)

---

_{Reviewed @ 1e72e373
"Every system eventually becomes legacy."}