Revert forcing Agent version to be 8.16.0

[ycombinator]

This PR reverts the changes made in #5565 as it is now possible to create 9.0.0-SNAPSHOT deployments on ESS prod in the CFT region.

Resolves #5567

[mergify]

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

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

[mergify]

backport-v8.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 and remove the backport-8.x label.

[jlind23]

@michel-laterman it looks there is still an issue while communicating with fleet-server which is:
`

(linux-amd64-ubuntu-2404-fleet) Test output (sudo) (stdout): Error: fail to enroll: fail to execute request to fleet-server: status code: 400, fleet-server returned an error: UnsupportedVersion, message: version is not supported 
`

Do you know where this could be coming from? This happened on the extended runtime leak tests

[ycombinator]

The issue seems to be that we are still creating an 8.16.0 deployment in ESS for some reason: https://buildkite.com/elastic/elastic-agent-extended-testing/builds/3296#01924d3f-01d4-4525-ae48-c1ddb2ded469/144-348

>>> Creating cloud stack 8.16.0-SNAPSHOT [stack_id: 8160-SNAPSHOT]
--
  | >>> Created cloud stack 8.16.0-SNAPSHOT [stack_id: 8160-SNAPSHOT, deployment_id: b463730b54b2456281dec88c341576a8]
  | >>> Waiting for cloud stack 8.16.0-SNAPSHOT to be ready [stack_id: 8160-SNAPSHOT, deployment_id: b463730b54b2456281dec88c341576a8]

[ycombinator]

@pchila just want to double check that this change makes sense in this PR, especially because the reverse change was not part of the original PR: #5565. Thanks.

[pchila]

This normally is handled by the automation ---> #5588
since it would be a chicken and egg problem I guess we can update .package.version here and close the other PR once we merge this

[ycombinator]

Now a bunch of tests are failing CI with:

        	Error:      	Received unexpected error:
        	            	failed to find a previous minor on the version list

I'm working on a fix for this issue and will add it to this PR.

[pchila]

Now a bunch of tests are failing CI with:

        	Error:      	Received unexpected error:
        	            	failed to find a previous minor on the version list

I'm working on a fix for this issue and will add it to this PR.

This happens because in our test where we look for a previous minor (still not convinced it is really needed) we don't handle the case of a new major with minor and patch version set to 0.

This would be a good opportunity to get rid of GetPreviousMinor() if possible

[elastic-sonarqube]

Quality Gate passed

Issues
0 New issues
0 Fixed issues
0 Accepted issues

Measures
0 Security Hotspots
No data about Coverage
0.0% Duplication on New Code

See analysis details on SonarQube

[rowlandgeoff]

LGTM as long as CI passes 😄