feat(NEXUS-1447): adopt shared renovate-config preset (slim extends-only)

[corinnesollows]

Adopt shared Renovate config preset (NEXUS-1447)

This repo is part of the org-wide rollout of nestoca/renovate-config — a shared Renovate preset that standardises dependency update behaviour across all nestoca/* repos.

What the shared preset provides

Preset	What it does
github>nestoca/renovate-config	Security-only baseline: all updates disabled by default; vulnerability alerts on
:internal-docker	Internal nesto docker images (Artifact Registry) bumped weekly on Mondays
:monthly-internal-gomod-bumps	github.com/nestoca/** Go deps bumped monthly in one grouped PR
:self-exclusion(<repo>)	Prevents self-bump PRs when this repo tags a new release

Repo-specific notes

Previously disabled all updates. The shared preset's security-only baseline plus the two required presets gives the right behaviour.

Behaviour change summary

• External Go deps: security-only (no proactive PRs) — same as before
• Internal nestoca/* Go deps: monthly grouped PR via shared preset
• Internal docker images: weekly via shared preset
• Vulnerability alerts: on (unchanged)

Part of NEXUS-959 · Batch 7 NEXUS-1447

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Note

Low Risk
Config-only change to Renovate automation; no application runtime or security logic is modified, only which dependency update PRs Renovate may open.

Overview
Renovate config is relocated and replaced with the org shared preset instead of the old root renovate.json that disabled all package updates while leaving vulnerability alerts on.

The new .github/renovate.json only extends nestoca/renovate-config (security-only baseline), plus internal Docker weekly bumps, monthly grouped internal Go module bumps for github.com/nestoca/**, and self-exclusion for this repo (joy) so Renovate does not open self-bump PRs on its own releases.

Behaviour shift vs the previous file: external deps stay security-only; internal nesto Go deps and internal docker images now get scheduled proactive PRs from the shared presets.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feat/NEXUS-1447/joy-slim-renovate

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