Improve the patch release workflow

[isaacaflores2]

The current patch release workflow is a bit confusing and generally results in a developer making manual changes versus relying on the automation.

Goals

1. Update the release workflow to avoid having to provide X.Y.Z+1 i.e for the 8.18.7 we should not have to pass 8.18.8. This is most likely due to update-version expecting the next release version here. A possible solution is listed below:
   • We already calculate X.Y.Z+1 here under the NEXT_RELEASE var: https://github.com/elastic/apm-server/blob/main/release.mk#L47
   • Example assuming the release is for 8.18.7 (env var RELEASE_VERISON)
     • This var is used to create the new branch update-8.18.8. But we pass in the RELEASE_VERISON to update-version https://github.com/elastic/apm-server/blob/main/release.mk#L47
     • So this should be updated to : $(MAKE) update-version VERSION=$(NEXT_RELEASE) ?
2. The automation creates a misleading branch. Example for the 8.18.7 release, the automation will create a branch update-8.18.8 to change all necessary files to the next version 8.18.8. The branch name implies the we are updating 8.18.8 versus changing files to bump to 8.18.8. This should help avoid any confusion on the purpose of the branch, especially for anyone not familiar with the details of the release workflows.
3. Update release documentation to clarify instructions

[isaacaflores2]

This PR attempts to clarify some instructions for 8.x patch releases: https://github.com/elastic/apm-server/pull/18634/files