Fix the upgrade notes of the Gerrit v3.15.0 release notes

The upgrade instructions were carried over from v3.14 and did not
match v3.15:
- The zero-downtime upgrade and the downgrade also cover v3.13
- Detail the schema 185 -> 187 migrations (186 is a no-op) and why
  the Changes index v90 needs a full reindex
- Add the Java 25 prerequisite, the missing init step to the offline
  upgrade and an online upgrade with limited downtime
- Explain that gerrit.experimentalRollingUpgrade is needed during a
  rolling upgrade, as v3.13/v3.14 refuse to start on schema 187
- Add a Downgrade section
- List two 3.15 bugs as known issues

Change-Id: I2cf67cf61da11cfe21ccda71ec91dc88b77f09b9