refactor(docs,blog): last update timestamp should be in milliseconds instead of seconds

[slorber]

Motivation

The last updated at timestamp read should be in milliseconds instead of seconds

Dealing with milliseconds is more convenient in JavaScript, and we shouldn't need to constantly convert it.

The only reason it was in second is because historically reading from git gives us a value in seconds, so let's convert it to ms at the root just after reading it from Git. This simplifies all the code down the line.

Test Plan

Unit tests + preview

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[github-actions]

⚡️ Lighthouse report for the deploy preview of this PR

URL	Performance	Accessibility	Best Practices	SEO	PWA	Report
/	🟠 67	🟢 98	🟢 96	🟢 100	🟠 88	Report
/docs/installation	🟠 69	🟢 96	🟢 100	🟢 100	🟠 88	Report
/docs/category/getting-started	🟠 75	🟢 100	🟢 100	🟢 90	🟠 88	Report
/blog	🟠 69	🟢 100	🟢 100	🟢 90	🟠 88	Report
/blog/preparing-your-site-for-docusaurus-v3	🟠 63	🟢 96	🟢 100	🟢 100	🟠 88	Report
/blog/tags/release	🟠 69	🟢 100	🟢 100	🟠 80	🟠 88	Report
/blog/tags	🟠 75	🟢 100	🟢 100	🟢 90	🟠 88	Report

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Build	Status	Details	Updated (UTC)
default (Inspect)	✅ No change detected	-	Mar 18, 2024, 7:10 PM

[github-actions]

Size Change: 0 B

Total Size: 992 kB

ℹ️ View Unchanged

Filename	Size
website/.docusaurus/globalData.json	75.4 kB
website/build/assets/css/styles.********.css	114 kB
website/build/assets/js/main.********.js	765 kB
website/build/index.html	37.9 kB

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