Every datasource that caches its releases hand-wrote the same
`withCache({ namespace: `datasource-${id}`, ... }, () => this._getReleases(config))`
wrapper, repeating the namespace literal and using underscore-prefixed
method names that `docs/development/best-practices.md` discourages.
Add `Datasource.cacheNamespace` (derived from the datasource id) and a
protected `Datasource.cached()` helper that defaults `namespace` to it,
exporting `CachedOptions` from `with-cache.ts`. Migrate 29 datasources
onto the helper, dropping their namespace literals, the static
`cacheNamespace` field in `github-digest`, and renaming the wrapped
`_getX` methods to `fetchX`.
Behaviour change: `golang-version` used the constant cache key `'all'`
while declaring `customRegistrySupport = true`, so a custom registryUrl
shared the cache entry with the default one. The key is now
`${config.registryUrl}`, which invalidates existing entries once.
Sebastian Poxhofer