Upgrade IKVM to OpenJDK 8u492

[Copilot]

Bumps the targeted OpenJDK version from 8u472 to 8u492 (b08, GA tagged upstream on 2026-04-06).

Changes

• Directory.Build.props: OpenJdkUpdateVersion 472 → 492

Pending

ext/openjdk submodule update is blocked on ikvmnet/jdk8u not yet having an ikvm-jdk8u492-b08 branch. Once that branch exists, the submodule needs to move from 336276d5af26a2f41055899fa1e62d527482edad (ikvm-jdk8u472-b08) to its tip.

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✅ wasabii
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[wasabii]

Superseded by #743.

This PR moved OpenJdkUpdateVersion to 492 but left ext/openjdk where it was, so it did not actually upgrade the JDK. Upstream has released twice since, so rather than finish this one against 492, #743 goes to the current GA, 8u504-b01, and moves the submodule to ikvm-jdk8u504-b01 on ikvmnet/jdk8u. Ancestry is linear, so 492 and 502 are included.

Closing in favour of that.