docs: stop the README from naming which release is current

[kkdev92]

Removes the two sentences that said which version was current, and pins the
versions that remain with a test.

Why

Both sentences were wrong within a day of being written:

• `4.1.0` is the current release
• `4.1.0` is the current release and holds `latest` on npm

4.1.1 shipped and neither moved. The npm badge at the top of the README already
renders the current version, and it is generated — so the prose was duplicating
a fact it had no way to keep.

An earlier change rewrote these to name the line (4.1, 4.x) instead of the
patch. That only lengthens the interval: 4.1 goes stale at 4.2 and 4.x at
5.0. The sentences are gone instead. What is left says what does not move — that
2.x continues on v2-maintenance, and that the CHANGELOG is where per-release
detail lives.

The versions that stay are requirements, so they are pinned

A reader deciding whether to adopt does need the floors. Those stay, and a new
suite holds them to the manifest:

• the VS Code and Node floors quoted in the documentation are the strings
  engines declares
• no other VS Code version is named on any documented page
• @types/vscode has not drifted away from engines.vscode — the README says
  the two move together, and this is that sentence as a check
• no page claims to know which release is current

This follows what tests/readme-samples.test.ts already does for documented
code: the same argument, applied to the documented numbers.

Verification

Each assertion was checked by breaking what it guards and confirming it fails —
a test that only ever passes proves nothing:

Change made	Caught by
README's VS Code floor set to 1.133.0	floor match, and "no other version named"
README's Node floor set to >=20.0.0	Node floor match
A stray 1.125.0 added to the README	"no other version named"
@types/vscode set to ~1.120.0	types-track-the-floor
The previous README restored	"leave currency to the badge", naming both sentences

Full suite: 1087 tests in 73 files. No package content changes beyond the
README.

🤖 Generated with Claude Code