fix: keep current index version at 8

[marcelo-cjl]

What

Keep the default/current index version at 8 while leaving version 11 available as the maximum supported nullable id-map format.

Also update nullable emb-list tests so MUVERA/LEMUR scenarios explicitly use the emb-list meta v2 minimum version instead of inheriting the default current version.

Why

#1768 correctly introduced the nullable/version-gated format, but making it the default current version would publish that development format by default. Restoring current_version to 8 preserves existing default index behavior.

After restoring current_version, the nullable MUVERA/LEMUR emb-list UTs must opt into version 11 because their serialized metadata is intentionally rejected for versions below the emb-list meta v2 gate.

Related to #1687.

Validation

• git diff --check
• make BUILD_DIR=/home/ubuntu/skills/issue_review/workspaces/knowhere_pr_1776_main/build_ut_asan WITH_UT=True WITH_ASAN=True
• build_ut_asan/Release/tests/ut/knowhere_tests "Nullable IVF_FLAT MUVERA/LEMUR emb-list APIs rebuild list ids after reload"
• build_ut_asan/Release/tests/ut/knowhere_tests "Nullable native matrix covers every IndexNode operation"

[sre-ci-robot]

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This pull-request has been approved by: marcelo-cjl

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