Add JDK 11+ requirement to java-sdk README

[Andrushika]

Add JDK 11+ requirement to java-sdk README

Why

I was testing the Java SDK 1.0.0-beta1-rc3 release candidate following java-sdk/README.md and encountered an incompatibility with the JDK: the build failed because my system JDK was 8, but it requires JDK 11 or later. The docs did not mention this requirement.

What

Add one line requirement description in README.md under the Building the SDK section.

No tests are needed.

Was generative AI tooling used to co-author this PR?

• Yes (please specify the tool below)

---

• Read the Pull Request Guidelines for more information. Note: commit author/co-author name and email in commits become permanently public when merged.
• For fundamental code changes, an Airflow Improvement Proposal (AIP) is needed.
• When adding dependency, check compliance with the ASF 3rd Party License Policy.
• For significant user-facing changes create newsfragment: {pr_number}.significant.rst, in airflow-core/newsfragments. You can add this file in a follow-up commit after the PR is created so you know the PR number.

[phanikumv]

Looks good

[jason810496]

I'm super sorry that I saw #69691 before this one and just merged the #69691 right after the review.

Unfortunately, I need to close this one.
Thanks for the fix again!

[jason810496]

Oh wait, perhaps we can move the (see the toolchain in buildSrc/src/main/kotlin/airflow-jvm-conventions.gradle.kts) into the same paragraph that #69691 introduced. So that we don't need to close this one.

Thanks!

[Andrushika]

No worries! I have seen #69691, and it is more comprehensive than my PR. I have rebased and added the toolchain reference line. Thanks for addressing this problem so quickly!