BUILD: Target Java 8 for the published library.

[oleksandrlazarenko-pi]

Summary

Lowers the compile target of the shipped library from Java 21 to Java 8, so
the artifact can be consumed by projects on a Java 8 baseline. Test sources keep
using modern language features and are compiled at a newer release. One Java 11
API call in Io.java is replaced with its Java 8 equivalent.

Motivation

We'd like to build on owid-java from SDKs that still target Java 8 (in our
case the 51Degrees pipeline-java). The library's source is almost entirely
Java 8-compatible already — only the build target and a single JDK 11 API stand
in the way. This change makes the published jar usable on Java 8 runtimes
without affecting behaviour or the development experience.

Changes

• pom.xml — set maven.compiler.release to 8 for the main sources.
  Added a default-testCompile execution at release 17, because the test
  sources use records (FixturesTest) and List.of(...), which are not part of
  the shipped artifact.
• Io.java — replaced the three ByteArrayOutputStream.writeBytes(byte[])
  calls (added in Java 11) with write(b, 0, b.length). This was the only JDK
  9+ API used in the main sources.

Compatibility & behaviour

• Behaviour is unchanged. ByteArrayOutputStream.write(byte[], int, int)
  writes the same bytes and, unlike the inherited OutputStream.write(byte[]),
  does not declare IOException, so no signatures or call sites change.
• API surface is unchanged — no public types or methods added, removed, or
  modified.
• The published bytecode moves to class file version 52 (Java 8).

Building

• Building still requires a modern JDK (17+) because the tests compile at
  release 17. Only the main artifact targets Java 8.
• CI (ci.yml) runs mvn test on JDK 21 across ubuntu + windows; both pass.
  Locally: Owid.class is major version 52, and an independent
  javac --release 8 of the main sources compiles cleanly (no Java 9+ API).

Notes for reviewers

• If you'd prefer the whole module (tests included) to build on JDK 8, the two
  test-only usages (record Fixtures, List.of) can be backported as well —
  happy to fold that in if you want a single uniform release level.