Adopt Java 25 as JGit's minimum supported version (Eclipse IDE 2026-12) #292

Description

Track the JGit decision to raise the minimum Java version from 17 to 25
for the Eclipse IDE 2026-12 timeframe.

The purpose of this issue is to make the new minimum supported Java
version visible to the JGit community, confirm the release/versioning plan,
and coordinate with Eclipse Platform, EGit and downstream consumers.

Motivation

Eclipse Platform is preparing Java 25 adoption work for the 2026-12
release train. The Platform meeting notes from 2026-09-02 say Java 25 was
already allowed for 2026-09 in SimRel and is now allowed for Platform,
with releng adaptation starting there.

"Allowed" is weaker than Java 25 being the minimum Platform baseline for
2026-12. This issue should track that decision explicitly: JGit should
raise its own minimum only if the project agrees that the Eclipse IDE /
EGit release train requires or accepts a Java 25 minimum.

If JGit does make that move, doing it now keeps the project aligned with
Eclipse IDE 2026-12 planning instead of carrying a different Java baseline
into the release cycle.

Scope

The compatibility decision is:

• Java 25 becomes the minimum supported Java version for JGit.
• Java 17 is no longer a supported runtime baseline for the affected JGit
  release line.
• Build files, bundle metadata, IDE metadata and CI change only as
  consequences of that baseline decision.

Compatibility / Versioning

Raising the minimum Java runtime from 17 to 25 is a breaking change for
JGit consumers. The project should decide whether this belongs in JGit
8.0.0 rather than the current 7.9.0-SNAPSHOT line.

This is independent of the source changes being behavior-preserving: the
compatibility break is the runtime baseline, not the code.

Downstream coordination is needed at least for:

• EGit / Eclipse IDE, which consumes JGit directly
• Gerrit, which already builds on Java 25
• other JGit embedders that still require Java 17 or Java 21

Merge / Release Preconditions

Before merging the baseline change, confirm:

• Eclipse Platform / SimRel 2026-12 has a target platform that resolves
  JavaSE-25
• EGit can consume the Java 25 JGit bundles in the intended release train
• JGit CI agents have JDK 25 for the relevant Maven, Tycho and Bazel
  builds
• the project agrees whether the first Java 25-minimum release is 8.0.0

Alternatives considered

• Update minimum supported Java version to Java 21 instead.

Additional context

• An implementation is already under review:
  https://review.gerrithub.io/c/eclipse-jgit/jgit/+/1241664
• Eclipse SimRel Java 25 discussion:
  Meeting Minutes for Planning Council 2026-09-02 eclipse-simrel/.github#98
• Eclipse Platform 2026-09-02 meeting notes (Java 25 policy for Platform
  in 2026-12)