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

### Feature Freeze for `8.18` and `9.0`
- [x] branch out an `8.18` branch from `8.x`
- [x] change version in `8.x` branch to `8.19`
- [x] remove automatic backport label to `8.x`
- [x] create label `backport-8.18` which needs to be set **manually** for any changes that should be backported to `8.18`.
- [ ] create label `backport-8.19` which needs to be set **manually** for any changes that should be backported to `8.19`.

### Feature Freeze for `8.19`
- [ ] rename `8.x` branch to `8.19`

[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.

[raultorrecilla]

moving this to it-105

[simitt]

@v1v with the introduction of an 8.18 release in sync with the 9.0 release and another release 8.19 afterwards (in sync with 9.1), we also need to adapt the automation listed in this task. I updated the description above, please reach out if anything is unclear.

[raultorrecilla]

in progress, moving it from it105 to it106

[v1v]

Some thoughts about the tasks in Feature Freeze for 8.18, 9.0 and 8.19:

• branch out an 8.18 branch from 8.x - Automated 🤖 (see run-minor-releases)
• change version in 8.x branch to 8.19 - Automated 🤖 (see run-minor-releases)
• remove automatic backport label to 8.x - Manual ✋
• create label backport-8.18. - Automated 🤖 (see run-minor-releases)
• create label backport-8.19. - Automated 🤖 (see run-minor-releases)
• rename 8.x branch to 8.19 - Manual ✋

In addition, I'm working on supporting major release automation in main for 9.0.0, see https://github.com/elastic/apm-server/pull/15329

[raultorrecilla]

moving it from it105 to it106

[kruskall]

This has been completed. Closing.