doc: update hydrogen lts status on readme

Given that v18 is turning LTS in v18.12.0 this change updates the
initial table located in the README of the repo in order to mark v18 as
being an LTS version along with adding its codename reference.

Signed-off-by: Ruy Adorno <ruyadorno@google.com>
Refs: nodejs/node#45100

Author: ruyadorno

README.md

@@ -6,7 +6,7 @@
 | :--:     | :---:               | :---:        | :---:          | :---:            | :---:             | :---:                     |
 | [14.x][] | **Maintenance**     | [Fermium][]  | 2020-04-21     | 2020-10-27       | 2021-10-19        | 2023-04-30                |
 | [16.x][] | **Maintenance**     | [Gallium][]  | 2021-04-20     | 2021-10-26       | 2022-10-18        | [2023-09-11][nodejs16eol] |
-| [18.x][] | **Current**         |              | 2022-04-19     | 2022-10-25       | 2023-10-18        | 2025-04-30                |
+| [18.x][] | **LTS**             | [Hydrogen][] | 2022-04-19     | 2022-10-25       | 2023-10-18        | 2025-04-30                |
 | [19.x][] | **Current**         |              | 2022-10-18     | -                | 2023-04-01        | 2023-06-01                |
 | 20.x     | **Pending**         |              | 2023-04-18     | 2023-10-24       | 2024-10-22        | 2026-04-30                |
 
@@ -149,6 +149,7 @@ the discretion of the Release working group.
 [Erbium]: https://nodejs.org/download/release/latest-erbium/
 [Fermium]: https://nodejs.org/download/release/latest-fermium/
 [Gallium]: https://nodejs.org/download/release/latest-gallium/
+[Hydrogen]: https://nodejs.org/download/release/latest-hydrogen/
 [JSON]: schedule.json
 [nodejs16eol]: https://nodejs.org/en/blog/announcements/nodejs16-eol/
 [4.x]: https://nodejs.org/download/release/latest-v4.x/