enhance: bump knowhere to v3.0.8

[foxspy]

Move the knowhere pin from 6a3b7df1 to the v3.0.8 tag (main at d85f7080, zilliztech/knowhere#1780), which bumps the cardinal references to v3.0.5 (v2) and v2.5.110 (v1).

The stale comment above the pin is updated to describe the new one.

Heads-up for reviewers: this range also brings in zilliztech/knowhere#1673 "support nullable external id mapping", which the previous pin deliberately excluded.

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[mergify]

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❌ CI Loop Results d5db8fa

Stage	Result	Duration	Tests
✅ Build	SUCCESS	15.9min	-
✅ Code-Check	SUCCESS	11.6min	-
✅ UT-Integration	SUCCESS	25.1min	-
✅ UT-GO	SUCCESS	22.3min	-
❌ UT-CPP-Cov	FAILURE	56.8min	8588 total, 8587 passed, 1 failed

Total: 79min | Pipeline | Artifacts
Diff Coverage: CPP 84.8% (434 hit, 78 miss, 512 measurable lines, 796 unmeasured)
Total Patch Coverage: 84.8% (434/512 measurable lines, 796 unmeasured)

Failed Test Logs:

• UT-CPP-Cov: view log

[foxspy]

Closing: superseded, and merging it now would be a regression.

master and 3.0 both already pin knowhere d7cfd888, not the 6a3b7df1 this PR was written against. The pin moved twice since:

• 08-19 77ebd82e0e (enhance: move nullable vector mapping into index layer #50524, move nullable vector mapping into index layer) → d85f7080
• 08-22 57ea5fff5e (fix: update Knowhere for stable IndexNode ABI #52754, stable IndexNode ABI) → d7cfd888

So this PR's intent — get onto a knowhere revision carrying cardinal v3.0.5 / v2.5.110 — is already satisfied, and d85f7080 is now 3 commits behind the current pin. Merging would drop:

• d7cfd888 fix: keep IndexNode ABI stable across Cardinal builds (fix: keep IndexNode ABI stable across Cardinal builds zilliztech/knowhere#1786)
• 8c864b5f fix: update SVS SQ8 storage enum (fix: update SVS SQ8 storage enum after Faiss 1.15.0+ upgrade zilliztech/knowhere#1784)
• 03b845e3 enhance: skip fully filtered windows in sindi sparse search (enhance: skip fully filtered windows in sindi sparse search zilliztech/knowhere#1782)

The concern in the original description — that this bump re-introduces zilliztech/knowhere#1673 — is also moot: 7cc9d4be is already an ancestor of the current pin (0 behind / 15 ahead).

If a tag-form pin is still wanted for the 3.0 branch, it should be a fresh tag cut at d7cfd888 or later, not v3.0.8 (= d85f7080).