[ci] Enable dependabot Helm version updates

[rogercoll]

The elastic-agent repo ships multiple Helm configurations for a variety of Helm Charts, as there might be breaking changes between Helm versions, we should aim to catch those as soon as possible. Automating the bump of the Helm version used in our tests should help to notice incompatibilities:

• https://github.com/elastic/elastic-agent/blob/main/.github/dependabot.yml
• https://github.com/elastic/elastic-agent/blob/main/go.mod#L106