apmpackage: update format_version

We should update the integration package to the latest package-spec version:
https://github.com/elastic/apm-server/blob/078b8da74881c55dd9e8407bd927d83afa2d1e12/apmpackage/apm/manifest.yml#L1

There will be additional changes needed, but I don't know what they are -- we'll find out when we attempt this.

Updating to 2.0.0+ will enable additional validations, such as https://github.com/elastic/package-spec/pull/413.

Start with apmpackage/apm/manifest.yml at the referenced format_version line and compare it with the latest package-spec version. Run the package-spec validations to identify the additional required changes; done means the integration package uses the newer version and passes those validations.

Written by the indexing model from the issue text.