docs: update stale Go version requirement and prometheus-operator links

[archy-rock3t-cloud]

• Change is not relevant to the end user.

Changes

Update Go requirement from 1.18+ to 1.26+ to match go.mod. Update coreos/prometheus-operator links to prometheus-operator/prometheus-operator. Add missing workflows to Go version change guide, fix .promu.yaml → .promu.yml.

Verification

Checked go.mod and GitHub org redirect.

[archy-rock3t-cloud]

Expanded scope to also fix 3 dead external links that were causing the Documentation check to fail across multiple thanos PRs:

• grafana.com/docs/loki/latest/clients/promtail/configuration (404 — Loki docs reorganized; Promtail is EOL as of March 2026 and the /clients/... paths no longer exist on /latest/. Pinned to versioned archive: grafana.com/docs/loki/v3.5.x/send-data/promtail/configuration)
• grafana.com/docs/loki/latest/clients/promtail/scraping (404 — same redirect to versioned archive)
• docs.lightstep.com/docs/create-and-use-access-tokens (403 — replaced with /docs/create-and-manage-access-tokens, which is the current "Manage access tokens" page on the post-acquisition Cloud Observability docs)

Verified replacement URLs return HTTP 200 via curl -I -L. make check-docs runs cleanly on the changed lines locally (no mdox link errors).

Also included a small fix on top: docs/components/receive.md had stale mdox-exec output (the snippet from pkg/receive/handler.go lines 1068-1078 had been updated upstream but the doc wasn't regenerated). This was failing the Documentation check independently across PRs.

This should also unblock the docs check on companion PRs #8734, #8737, #8738, #8739, #8743 once they pick up the rebased main.