Upgrade webpack v4 → v5

[victorlin]

Description of proposed changes

webpack 4 does not support Node.js v18 without the --openssl-legacy-provider option.

Related issue(s)

Support for Node.js 18 (#1553) relies on this webpack upgrade.

Testing

• [x] Checks pass
• [ ] nextstrain.org preview PR looks good
• [x] auspice develop and hot-reloading works (comment)
• [ ] nextstrain.org auspice-client works (comment)

[jameshadfield]

👍 thanks Victor -- I'll re-review this before the end of the week

[victorlin]

Split the changes into 3 PRs:

• Upgrade webpack v4 → v5 (this PR)
• #1559 (this should be good to merge)
• #1560 (this relies on the webpack upgrade)