Update logstash_releases.json for new branching strategy

[jsvd]

See naming rational in https://github.com/logstash-plugins/.ci/pull/63#issue-2597373955 After 8.16 is GA, the "releases" entry should become:

  "releases": {
    "7.x": "7.17.24",
    "8.n": "8.16.0",
    "8.n-1": "8.15.3"
  },

[robbavey]

@jsvd I updated the commit comment to include 7.current instead of 7.latest

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