…st release link (#27260)

## Problem

The interactive releaser (`scripts/releaser`) renders the "Latest
Release"
cell of the release calendar with a doubled version prefix, e.g.
`[vv2.35.0](.../tag/v2.35.0)`.

`version.String()` already returns a `v`-prefixed string (e.g.
`v2.35.0`),
but `updateCalendar` wrapped it in a `"[v%s]"` template, so the link
label
gained a second `v`. The tag URL was already correct because release
tags
carry the `v` prefix.

## Fix

Drop the extra `v` from the label template (`"[v%s]"` → `"[%s]"`). The
URL is
unchanged.

- Label before: `[vv2.35.0]`
- Label after: `[v2.35.0]`

## Test

Added `scripts/releaser/v1/docs_test.go`:

- `TestUpdateCalendarLatestReleaseVersionPrefix` asserts the
`LatestRelease`
cell for a matching row on both a patch and a minor release. It fails on
the
  old code (`[vv2.35.x]`) and passes with the fix.
- `TestUpdateCalendarNotReleasedRowName` covers the `Not Released` →
`Mainline`
  promotion and the major.minor "Release name" link (patch omitted).

<details>
<summary>Investigation notes</summary>

- Entry path: `scripts/release.sh` → `go run ./scripts/releaser
--legacy` →
  `runRelease` → `promptAndUpdateDocs` → `updateReleaseDocs` →
`updateCalendarFile` → `updateCalendar` (`scripts/releaser/v1/docs.go`).
- Root cause in `updateCalendar`: `fmt.Sprintf("[v%s](%s)",
newVer.String(), ...)`
  combined with `version.String()` returning `v%d.%d.%d`.
- Only the link label was affected; the `releaseTagURLFmt` URL was
correct
  because tags are `v`-prefixed.
- The standalone `scripts/update-release-calendar.sh` is a separate
implementation and is not affected (it strips the `v` before re-adding
one).
- Companion PR for `release/2.35` (file `scripts/releaser/docs.go`):
#27259.

</details>

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