chore: サポートする Node.js のバージョンを `v20` に固定

[m1sk9]

Details of implementation (実施内容)

OreOreBot2 がサポートする Node.js バージョンを v20 (現行の最新 LTS) に固定しました.

[cloudflare-workers-and-pages[bot]]

Deploying with    Cloudflare Pages

Latest commit:	f3ef639
Status:	✅  Deploy successful!
Preview URL:	https://278bc426.oreorebot2.pages.dev
Branch Preview URL:	https://upgrade-support-nodejs.oreorebot2.pages.dev

View logs

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (2a71b73) 56.71% compared to head (f3ef639) 56.71%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1257 +/- ## ======================================= Coverage 56.71% 56.71% ======================================= Files 103 103 Lines 7048 7048 Branches 517 517 ======================================= Hits 3997 3997 Misses 2535 2535 Partials 516 516 ``` | [Flag](https://app.codecov.io/gh/approvers/OreOreBot2/pull/1257/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=approvers) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/approvers/OreOreBot2/pull/1257/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=approvers) | `56.71% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=approvers#carryforward-flags-in-the-pull-request-comment) to find out more.

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.