Migrate to Google Analytics 4 (GA4) for fluentd.org

This is part of a CNCF-wide effort to upgrade project websites to GA4 since **Google has deprecated Universal Analytics** (UA). For more details, see:

- https://github.com/cncf/techdocs/issues/108, including reference to:
- A suggested [migration process](https://github.com/cncf/techdocs/blob/main/docs/analytics.md#migrating-to-google-analytics-4),
which we've used to [migrate a few CNCF projects](https://docs.google.com/spreadsheets/d/1Mx4LhdI2Un-rvGMI73SlHxQH9D2HABAJclMB3dd6lnA) already.

### Tasks

- [x] Create a GA4 site tag under the **CNCF projects** account.
The new GA4 stream measurement ID is: **`G-93T2VC0VBC`**
- [x] Main website - `UA-82811140-41`:
- [x] [Connect] UA site tag from the GA4 site tag.
- [x] Use [gtag.js] library to register GA4 ID - fluent/fluent-bit#241
- [x] Confirm that the GA4 site tag is receiving events
- [x] Confirm that the UA site tag, `UA-82811140-41`, is receiving events
- [ ] [docs.fluentd.org](https://docs.fluentd.org/) - `UA-181341719-4`, which CNCF doesn't currently have access to.
https://github.com/fluent/fluentd-docs-gitbook/issues/437
- [ ] [Connect] UA site tag from the GA4 site tag.
- [ ] Use [gtag.js] library to register GA4 ID

### Related

- For [fluentbit.io](https://fluentbit.io), see:
- https://github.com/fluent/fluent-bit-website-hugo/issues/22
- https://github.com/fluent/fluent-bit-docs/issues/1002

### Later

- Cleanup: there are still traces in the main site pages of the old GTM ID. Those scripts should be removed.

/cc @nate-double-u @caniszczyk @fujimotos

---

Related, background & prep-work:

- fluent/fluent-bit#100: apparently TD used `GTM-T7BJ3X` and the CNCF used `GTM-TFCP8WJ` though there is no trace of such a GTM ID at the CNCF.
- fluent/fluent-bit#234

[connect]: https://support.google.com/analytics/answer/9973999
[gtag.js]: https://developers.google.com/analytics/devguides/collection/gtagjs