USWDS-Site - In-page Nav: Update custom header offset

[mahoneycm]

Description

Changes to in-page nav in USWDS 3.1.2 will affect behaviors on site. As captured in testing (#2978), we'll need to update our custom offset to work with the new package.

Important

We'll need to update the header offset logic on our website to account for this change.

Installing this branch on Site did resolve the bug where nested headers would cause the window to scroll to the top of the page but due to our custom JS the offset was not calculated correctly.

Originally posted by @mahoneycm in uswds/uswds#5878 (comment)

[mejiaj]

This issue is small, let's cherry-pick the commit in #3039 and close.

@mahoneycm to look into some custom javascript that might affect this.

[mahoneycm]

Confirmed that sidenav.js and scroll-to-top-for-hash.js do not interfere with this work and do not need to be updated to prevent regression 👍