feat: drop backward compatible flag

[kruskall]

Motivation/summary

beats removed backward compatible flag support in elastic/beats#42117 but we're adding it back at https://github.com/elastic/apm-server/blob/main/internal/beatcmd/init.go#L57-L61We should remove the hack as well

Closes #15424

Checklist

• Update CHANGELOG.asciidoc
• Documentation has been updated

For functional changes, consider:

• Is it observable through the addition of either logging or metrics?
• Is its use being published in telemetry to enable product improvement?
• Have system tests been added to avoid regression?

How to test these changes

Related issues

[mergify]

This pull request does not have a backport label. Could you fix it @kruskall? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-7.17 is the label to automatically backport to the 7.17 branch.
• backport-8./d is the label to automatically backport to the 8./d branch. /d is the digit.
• backport-8.x is the label to automatically backport to the 8.x branch.

[mergify]

backport-8.x has been added to help with the transition to the new branch 8.x.
If you don't need it please use backport-skip label.

[1pkg]

nit: there is no more point to have dedicated initFlags function just move cfgfile.Initialize() directly to init

[kruskall]

will cleanup in a followup PR