Prepare branches for 8.x and 9.0 releases

[simitt]

In anticipation of a major version upgrade to 9.0 , following changes need to be made to the branching strategy:

### Initial setup for `9.0`
- [x] branch out an `8.x` branch from `main` with version `8.16`
- [x] change version in `main` branch to `9.0`
- [x] automatically add `backport-8.x` label to all PRs against `main` branch, as any changes will likely go into `main` and `8.x` for a certain time (if label is manually removed, then no backport PR should be created).

### Feature Freeze for `8.16`
- [x] branch out an `8.16` branch from `8.x`
- [x] change version in `8.x` branch to `8.17`

### Feature Freeze for `8.17` 
- [ ] branch out an `8.17` branch from `8.x`
- [ ] change version in `8.x` branch to `8.18`
- [ ] adapt to automatically add `backport-8.18` label to all PRs against `main` branch, instead of `backport-8.x` label

### Feature Freeze for `9.0` 
- [ ] remove automatic backport label to `8.18`

[v1v]

I created an initial PR with some of the requirements mentioned above:

• https://github.com/elastic/apm-server/pull/13978

We, robots, are also tracking this effort in an internal GH issue. Glad we can collaborate on this together.

[simitt]

@v1v does the robots team have capacity to take care of the initial setup for 9.0 or should we plan for this? We need the branches to be ready by Sep 11th.

[v1v]

@v1v does the robots team have capacity to take care of the initial setup for 9.0 or should we plan for this? We need the branches to be ready by Sep 11th.

Yes, we can work on it - it's planned in our current and upcoming iteration.

[simitt]

@v1v I updated the description above to the latest changes to include 8.17 and 8.18 correctly.