Add release-progress badges: Next/Latest Release, Medium/Low priority (#6537)

* Add Next Release badge and Medium/Low priority progress badges

Adds a "Next Release" badge (in-development version from H5public.h)
and extends the existing Critical/High priority progress badges with
Medium (P2) and Low (P3) priority tracking, all fed from the same
GitHub Project #39 gist-based badge pipeline.

* Remove in-progress 2.0 entry from release schedule diagram

The Release Schedule chart is meant to show only past releases that
have reached end of life; the in-development series is now tracked
separately by the Release Progress badges. Regenerate the PNG from
the updated PlantUML source and clarify the README wording.

* Add Latest Release badge and milestone target date to Next Release

Fetches the most recent published release in the current major
version series from the GitHub Releases API (e.g. "2.1.1
(2026-03-23)") and exposes it as a new "Latest Release" badge.
Also looks up the target due date of the matching GitHub milestone
(e.g. "HDF5 2.2.0") and appends it to the existing Next Release
badge when one is set. Both lookups are independent of the
project-board query and degrade gracefully to "N/A" on failure.

release_docs/img/release-schedule.plantuml

@@ -36,10 +36,4 @@ Project starts 2023-01-01
 [1.14.6 (EOL)] displays on same row as [1.14.1]
 [1.14] is colored in #B187CF
 
-[2.0 Active] starts at 2024-09-30 and lasts 85 weeks
-[2.0.0] happens at 2025-09-30
-[2.1.0] happens at 2026-1-30
-[2.1.0] displays on same row as [2.0.0]
-[2.0 Active] is colored in #02BFA0
-
-@endgantt
+@endgantt