Stop exporting SMALL_THRESHOLD from Base.Sort

[LilithHafner]

It shouldn't be public as it's a quintessential implementation detail but this export makes it public via Base.Sort.SMALL_THRESHOLD.

Noticed when attempting to document it in #62026.

[LilithHafner]

@nanosoldier runtests()

[nanosoldier]

The package evaluation job you requested has completed - possible new issues were detected.
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❗ Packages that crashed

276 packages crashed on the previous version too.

✖ Packages that failed

8 packages failed only on the current version.

• Package has test failures: 2 packages
• Package tests unexpectedly errored: 2 packages
• Tests became inactive: 1 packages
• Test duration exceeded the time limit: 3 packages

1364 packages failed on the previous version too.

✔ Packages that passed tests

14 packages passed tests only on the current version.

• Other: 14 packages

5829 packages passed tests on the previous version too.

~ Packages that at least loaded

3553 packages successfully loaded on the previous version too.

➖ Packages that were skipped altogether

1 packages were skipped only on the current version.

• Package could not be installed: 1 packages

923 packages were skipped on the previous version too.

[LilithHafner]

@nanosoldier runtests(["SortingAlgorithms", "BiBufferedStreams", "Permanents", "BaytesOptim", "SurfaceCoverage", "HypersurfaceRegions", "Trixi", "Test"])

[nanosoldier]

The package evaluation job you requested has completed - possible new issues were detected.
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✖ Packages that failed

1 packages failed only on the current version.

• Package tests unexpectedly errored: 1 packages

1 packages failed on the previous version too.

✔ Packages that passed tests

6 packages passed tests on the previous version too.