Add support for PHP 8.3

[sacOO7]

• Update CI to support PHP 8.3.
• Update README for the same.
• Internal discussion -> https://ably-real-time.slack.com/archives/CURL4U2FP/p1700733173822189

┆Issue is synchronized with this Jira Task by Unito

[sync-by-unito]

➤ Automation for Jira commented:

The link to the corresponding Jira issue is https://ably.atlassian.net/browse/SDK-3960