[8.x](backport #5512) Update the regex used to identify release branches to include 8.x

[mergify]

What does this PR do?

Update the regex used to identify release branches

Why is it important?

Now we have 8.x which isn't matched by the current regex

Checklist

• My code follows the style guidelines of this project
• [ ] I have commented my code, particularly in hard-to-understand areas
• [ ] I have made corresponding changes to the documentation
• [ ] I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• [ ] I have added an entry in ./changelog/fragments using the changelog tool
• [ ] I have added an integration test or an E2E test

Disruptive User Impact

None

How to test this PR locally

• run the unit tests

go test -v -run TestGetReleaseBranches ./pkg/testing/tools/git

Related issues

• N/A

Questions to ask yourself

• How are we going to support this in production?
• How are we going to measure its adoption?
• How are we going to debug this?
• What are the metrics I should take care of?
• ...

---

This is an automatic backport of pull request #5512 done by [Mergify](https://mergify.com).

[elastic-sonarqube]

Quality Gate passed

Issues
0 New issues
0 Fixed issues
0 Accepted issues

Measures
0 Security Hotspots
No data about Coverage
No data about Duplication

See analysis details on SonarQube

[ycombinator]

Tests are failing CI on this PR because they are looking for a non-existent snapshot build, for example:

=== RUN   TestStandaloneUpgrade/Upgrade_8.14.4-SNAPSHOT_to_8.16.0-SNAPSHOT_(privileged)
    upgrade_standalone_test.go:68: 
        	Error Trace:	C:/Users/windows/agent/testing/integration/upgrade_standalone_test.go:68
        	            				C:/Users/windows/agent/testing/integration/upgrade_standalone_test.go:43
        	Error:      	Received unexpected error:
        	            	failed to prepare before exec: failed to find artifact URI for version 8.14.4-SNAPSHOT: failed to find snapshot information for version "8.14.4-SNAPSHOT": snapshot for version "8.14.4" not found
        	Test:       	TestStandaloneUpgrade/Upgrade_8.14.4-SNAPSHOT_to_8.16.0-SNAPSHOT_(privileged)
--- FAIL: TestStandaloneUpgrade/Upgrade_8.14.4-SNAPSHOT_to_8.16.0-SNAPSHOT_(privileged) (0.18s)

However, in order to make the tests not look for this non-existent build any more, we need to remove that version, 8.14.4-SNAPSHOT, from here (on the 8.x branch):

elastic-agent/.agent-versions.json

Line 5 in c671456

"8.14.4-SNAPSHOT",

That work is done by automation, which is the thing being fixed by this PR. So we have a 🐔 / 🥚 problem. As such, force merging this PR...