use correct property name for reh-web and web positronBuildNumber

[sharon-wang]

Description

• A follow-up to Provide CI builds of Positron Server for Linux #1618 and Fix up dependencies and build artifacts for reh-web build #4426
• Fixes the incorrect product.json property name by changing buildNumber to positronBuildNumber in the reh-web and web, resolving an issue where the build number always shows as 0 in dev and release builds for reh-web
• I just saw Positron Version: Server About dialog shows outdated information #2059 which is related, but this does not resolve that issue, which is specific to dev builds where product.json doesn't get injected. Note that the hardcoded product info in that code doesn't include positronBuildNumber, so it will display 0 by default.

Preview

QA Notes

See https://github.com/posit-dev/positron-builds/pull/49 for instructions on how to run a build.

The build's "About Positron" dialog should show the actual build number, which is the number of commits since the anchor commit in a release build.