fix(docs): keep only the latest patch per minor in the version selector and choose the default by semver

[Vulthil]

Summary

• When eng/update_versions.py registers a stable version slug, it now prunes every other stable entry sharing the same (major, minor) line from versions.json and deletes that entry's directory under --root. main and prerelease slugs are never pruned and never trigger pruning, and pruning never crosses minor lines (registering 1.0.3 does not touch 1.1.x).
• The site default is no longer set by --make-default directly. It is now always recomputed as the highest stable version present after pruning, falling back to main when no stable version exists. --make-default is still accepted (docs.yml passes it on every call) but is now advisory only, so a servicing release on an older minor line can no longer steal the site default away from a newer minor.

Behavior change to be aware of

Superseded patch docs directories are deleted from gh-pages when a newer patch in the same minor line is registered. Any deep link into a pruned patch's docs (e.g. /1.0.1/... after 1.0.2 ships) will 404. This is intentional — patch docs are strictly superseded — but is a visible change from today's behavior, where old patch directories accumulated forever.

Verification

No test harness exists for eng/, so this was verified by running the script against a scratch gh-pages tree for the scenarios called out in the acceptance criteria:

a) {main, 1.0.0, 1.0.1, default=1.0.1} + register 1.0.2

before: versions=[main, 1.0.1, 1.0.0], default=1.0.1, dirs=[1.0.0, 1.0.1, main]
after:  versions=[main, 1.0.2],        default=1.0.2, dirs=[main, 1.0.2]   redirect -> ./1.0.2/

b) {main, 1.0.2, 1.1.0, default=1.1.0} + register 1.0.3 --make-default true

before: versions=[main, 1.1.0, 1.0.2], default=1.1.0, dirs=[1.0.2, 1.1.0, main]
after:  versions=[main, 1.1.0, 1.0.3], default=1.1.0, dirs=[1.1.0, main, 1.0.3]  redirect unchanged -> ./1.1.0/

c) register main

before/after identical: versions=[main, 1.0.1, 1.0.0], default=1.0.1

d) register 1.2.0-preview

after: versions=[main, 1.2.0-preview(prerelease), 1.0.1, 1.0.0], default=1.0.1 (unchanged)

Backport to v1.0: yes -- #287