deps,test: restore month name in DateTimeFormat with calendar=iso8601

[JamBalaya56562]

Fixes: #63041

Bug

Intl.DateTimeFormat with calendar: 'iso8601' (or with the
-u-ca-iso8601 locale extension) silently drops the month part of the
output for dateStyle 'medium'/'long'/'full' and for any explicit
month option:

new Intl.DateTimeFormat('en-US', {
  dateStyle: 'full', timeStyle: 'long',
  timeZone: 'UTC', calendar: 'iso8601',
}).format(new Date('2024-09-09T08:00:00Z'));
// before: "2024  9, Monday at 08:00:00 AM UTC"   ← month gone
// after : "2024 September 9, Monday at 08:00:00 AM UTC"

The regression was introduced in v24.13.0 with the bundled ICU update.
Per Unicode TR35 the iso8601 calendar is gregorian with ISO
week-numbering and inherits gregorian symbols, but ICU's iso8601
resource bundle is intentionally empty and does not carry month-name
symbols, so MMMM/MMM expansions resolve to empty strings during pattern
construction.

Fix

In JSDateTimeFormat::CreateDateTimeFormat, derive a copy of the ICU
locale with ca=gregory and use it for the DateTimePatternGenerator
and SimpleDateFormat construction (and the extension-fallback retry).
The iso8601 Calendar instance is still produced from the original
locale and attached via adoptCalendar, so resolvedOptions().calendar
continues to report "iso8601" and ISO week-numbering semantics are
preserved.

Reproduction

Independently reproduced against an unmodified Node v26.1.0 baseline:
https://aletheia-works.github.io/vivarium/repro/node/63041/

Tests

• Added test/parallel/test-intl-iso8601-calendar-month.js covering
  dateStyle: 'full'/'long'/'short', explicit month: 'long', the
  explicit field skeleton path, and formatToParts month part. The
  test fails on the pre-patch tree and passes on this branch.
• make -j test-intl passes locally on Linux/x86_64.

[nodejs-github-bot]

Review requested:

• @nodejs/security-wg
• @nodejs/v8-update