Updated README.md and the timeline

[bmribler]

Note that release 4.0 is tentative and depends on funding.

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Important

Update release schedule and timeline for HDF4 in README.md and release-schedule.plantuml.

• Release Schedule Updates:
  • README.md: Updated release date for HDF4 3.1 to June 2025 and tentative release for HDF4 4.0 to June 2026.
  • release-schedule.plantuml: Extended duration of [4.4] to 78 weeks and updated [4.4.0] release date to June 30, 2026.
• Misc:
  • Clarified that release 3.1 is the last in the HDF4 3.x series in README.md.

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