[Build] Create new stream repositories in release preparation pipeline

[HannesWell]

Additionally add the previous release RC as first children immediately to the new I-build repository so that it isn't empty and the RC is used as baseline for the first builds of the new stream as also described in

• Preparation work for 4.37(2025-09) and open master for development #3070 (comment)

Follow-up on

• [Build] Add Jenkins pipeline to improve release preparation automation #3078

This is currently a draft as it requires some final testing.

[HannesWell]

Testing was successful and this is ready for submission.

@MohananRahul FYI, with this, when the 4.38 development cycle is prepared in late summer, this pipeline will also create the repositories

• https://download.eclipse.org/eclipse/updates/4.38
  as empty composite
• https://download.eclipse.org/eclipse/updates/4.38-Y-builds/
  as empty composite
• https://download.eclipse.org/eclipse/updates/4.38-I-builds/
  with a reference to the then specified RC2 build of the 4.37 release as only child of the composite

So that the problem described in #3070 (comment) ff should not occur.
I have also updated the release documentation that the new stream repos don't have to be created in a separate step and have removed the definition of the corresponding job.

As next step in this effort, I'll update the pruning of the composite repository so that #3123 does not happen anymore and the reference to the previous RC2 is automatically removed when the first I-builds of the then new stream have succeed.