Exclude PG 16 on macOS due to strchrnul compile error

What does this merge request do and why?

This MR adds a temporary workaround to the update:tool-versions Rake task to skip installing PostgreSQL 16 on macOS.

The build fails because strchrnul is only available starting in macOS 15.4.

Related to #2556 (closed)

How to set up and validate locally

1. Run gdk config set gdk.debug true
2. Run bundle exec rake update:tool-versions
3. Look for DEBUG: Combined tool versions content and make sure postgres does not include version 16

Click to expand

⛏️  DEBUG: Combined tool versions content:
python 3.11.11
poetry 1.8.5
golang 1.22.3 1.23.6 1.24.0 1.24.1
nodejs 20.18.0 20.12.2
markdownlint-cli2 0.17.1
vale 3.10.0 3.9.3
meson 1.7.0
ninja 1.12.1
protoc 21.12
golangci-lint 1.64.5
gitleaks 8.23.3
minio 2022-07-15T03-44-22Z
postgres 14.9
redis 7.0.14
rust 1.73.0
yarn 1.22.19
shellcheck 0.10.0

Impacted categories

The following categories relate to this merge request:

• gdk-reliability - e.g. When a GDK action fails to complete.
• gdk-usability - e.g. Improvements or suggestions around how the GDK functions.
• gdk-performance - e.g. When a GDK action is slow or times out.

Merge request checklist

• This MR references an issue describing the change.
• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise an issue to follow-up.
• Documentation added/updated, if needed.
• Announcement added, if change is notable.
• gdk doctor test added, if needed.