vecindex: enable vector indexing by default

[mw5h]

While vector indexing was in tech preview, it was disabled by default. Now that we are transitioning this feature to general availability, it needs to be enabled by default.

Epic: CRDB-52151
Release note (sql change): Vector indexing is now enabled by default.

[blathers-crl]

It looks like your PR touches production code but doesn't add or edit any test code. Did you consider adding tests to your PR?

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[cockroach-teamcity]

This change is 

[rytaft]

🎉

@rytaft reviewed 1 of 1 files at r1, all commit messages.
Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @mw5h)

[michae2]

Exciting!

@michae2 reviewed 1 of 1 files at r1, all commit messages.
Reviewable status: complete! 2 of 0 LGTMs obtained (waiting on @mw5h)

[mw5h]

bors r+

[mw5h]

TFTR!

[craig]

Build succeeded:

• check_generated_code
• linux_amd64_build
• local_roachtest_fips
• lint
• unit_tests
• local_roachtest
• docker_image_amd64
• acceptance
• linux_amd64_fips_build
• examples_orms

[blathers-crl]

Encountered an error creating backports. Some common things that can go wrong:

1. The backport branch might have already existed.
2. There was a merge conflict.
3. The backport branch contained merge commits.

You might need to create your backport manually using the backport tool.

---

💡 Consider backporting to the fork repo instead of the main repo. See instructions for more details.

error creating merge commit from 11f6cdd to blathers/backport-release-25.4-155536: POST https://api.github.com/repos/cockroachdb/cockroach/merges: 409 Merge conflict []

you may need to manually resolve merge conflicts with the backport tool.

Backport to branch 25.4.x failed. See errors above.

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