Update Go version references to 1.26.5 in development docs

Lachlan Grantrequested to merge
docs-fix-go-version-1.26.5 into main

What does this MR do?

`GO_VERSION` in `.gitlab/ci/_common.gitlab-ci.yml` was bumped to `1.26.5` by a Renovate commit, but `docs/development/_index.md` was not updated to match. This currently breaks the "docs: check development docs Go version" CI job on every pipeline run off `main` (e.g. https://gitlab.com/gitlab-org/gitlab-runner/-/jobs/15609638751).

This MR updates the Go version references in the development docs to `1.26.5` to match.

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