[RelEng] Compute release dates from provided SimRel dates

[HannesWell]

This draft demonstrates how a future, machine-readable representation of SimRel release/milestone dates can be leveraged to simplify the release preparation:

• Meeting Notes for Planning Council 2025-09-03 eclipse-simrel/.github#60

As the machine-readable representation is not yet implemented, e.g. the URL can still change.

[HannesWell]

This is now ready and just awaits the availability of

• Provide SimRel dates in machine-readable form and add details generator eclipse-simrel/.github#61

and ideally also the dates of the 2026-03 release (to simplify future testing of the release preparation pipeline, but that can also be worked around in testing).

[HannesWell]

This is now ready and just awaits the availability of

This was just submitted and is now available and thus is ready for submission. The verification build doesn't test this, but my manual testing was successful, thus I'm submitting this immediately.

@MohananRahul and @subyssurendran666, with this the number of input parameters for Prepare Next Development Cycle job is now drastically reduced.
Please let me know if anything is not clear. In general I plan a greater overhaul of the release documentation where this will also be explained in more detail.