Add main branch and update last minor in the release definition.

[mashhurs]

Description

We are currently using following format for the CI release definition. With the release section all good but to mitigate the future risks, we need to make sure we cover test cases with future snapshot versions. Current list doesn't cover the current.minor-SNAPSHOT (example, last release is 8.11.1 and snapshot is 8.11.2-SNAPSHOT). So, updating 8.x with curren minor and adding main would give us a coverage of testing plugins against future stack versions.

• AS IS:

{
    "releases": {
        "5.x": "5.6.16",
        "6.x": "6.8.23",
        "7.x": "7.17.15",
        "8.x": "8.11.1"
    },
    "snapshots": {
        "7.x": "7.17.16-SNAPSHOT",
        "8.x": "8.12.0-SNAPSHOT"
    }
}

• SUGGESTION

{
    "releases": {
        "5.x": "5.6.16",
        "6.x": "6.8.23",
        "7.x": "7.17.15",
        "8.x": "8.11.1"
    },
    "snapshots": {
        "7.x": "7.17.16-SNAPSHOT",
        "8.x": "8.11.2-SNAPSHOT", # keep consistent with versions.yml of the 8.x branch
        "main": "8.12.0-SNAPSHOT" # keep consistent with main branch (this might be 9.0.0 depending on release plan)
    }
}

[mashhurs]

Addressed by #15924