docs: bump README install Go version to 1.26.7

[imitater-dou]

Summary

README Build step still says Install Go 1.26.5, but tip go.mod / .tool-versions / Dockerfile use 1.26.7 (bumped in #23614 without updating README).

Repro

TIP=3e5d31a999baebaecc559c006133bd7530891a5d
curl -sL https://raw.githubusercontent.com/smartcontractkit/chainlink/$TIP/README.md | rg -n 'Install Go'
curl -sL https://raw.githubusercontent.com/smartcontractkit/chainlink/$TIP/go.mod | head -3
curl -sL https://raw.githubusercontent.com/smartcontractkit/chainlink/$TIP/.tool-versions | head -1

Change

README.md: Install Go 1.26.5 → Install Go 1.26.7.