feat!: upgrade Node.js minimum supported version to 18

[Kocal]

Hi, this PR upgrades pkg-dir ^7.0 to ^8.0, which reduces the dependencies tree and bandwidth consumption:

Package size report
===================

Package info for "find-cache-dir@5.0.0": 71 kB
  Released: 2023-08-18 23:47:06.336 +0000 UTC (53w5d ago)
  Downloads last week: 120,595 (0.34%)
  Estimated traffic last week: 8.6 GB
  Subdependencies: 8

Removed dependencies:
  - pkg-dir@7.0.0: 59 kB (82.96%)
    Downloads last week: 6,364,454 (N/A% from 7.0.0)
    Downloads last week from "find-cache-dir@5.0.0": 120,595 (N/A%)
    Traffic last week: N/A
    Traffic from "find-cache-dir@5.0.0": 8.6 GB (N/A%)
    Subdependencies: 6 (75%)

Added dependencies:
  + pkg-dir@8.0.0: 13 kB (18.48%)
    Downloads last week: 38,758 (N/A% from 8.0.0)
    Estimated traffic last week: N/A
    Subdependencies: 1 (12.5%)

Estimated new statistics:
  Package size: 71 kB → 18 kB (24.61%)
  Subdependencies: 8 → 3 (-5)
  Traffic with last week's downloads:
    For current version: 8.6 GB → 2.1 GB (6.5 GB saved)
    For all versions: 2.5 TB → 615 GB (1.9 TB saved)

Before:

After:

Since pkg-dir@^8.0 requires Node.js 18, we must requires Node.js 18 here too. This update is a breaking change ... or not, depending if dropping a non-maintained version is a BC or not.

Thanks :)