Switch strategy.matrix.series to JSON flow notation

[Copilot]

Collapse strategy.matrix.series entries from verbose YAML block notation to compact JSON flow mappings ({}). Array fields use [] to preserve join() compatibility.

Before:

series:
  - version: 20210902
    php: "8.1"
    liblzf: "3.6"
    redis:
      - "5.3.7"
      - "6.3.0"
    # ... 10+ lines per entry

After:

series:
  - {version: 20210902, php: "8.1", liblzf: "3.6", ..., redis: ["5.3.7", "6.3.0"], ...}

Net -85 lines. No semantic changes — stack, exclude, include, and all step references are untouched.

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