Allow setting default_ttl

[cosmastech]

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1️⃣ Why should it be added? What are the benefits of this change?
Our database had 1+ million cached entries with 'phpspreadsheet.' as the prefix. For all of them, their TTL was set to -1, so they never expired. It looks like the way Illuminate's Cache Repository works, it will rely on the library's default setting.

I doubt there's ever much of a reason to have a default TTL of forever on cell cache, but to avoid a big breaking change, I took the approach that I did.

2️⃣ Does it contain multiple, unrelated changes? Please separate the PRs out.
No

3️⃣ Does it include tests, if possible?
Yes.

4️⃣ Any drawbacks? Possible breaking changes?
I don't think there's any drawback here. It would seem to me that we probably would want a more sensible default other than null, but given that this is the current behavior and a minor release, I just left the default as null.

5️⃣ Mark the following tasks as done:

• Checked the codebase to ensure that your feature doesn't already exist.
• Take note of the contributing guidelines.
• Checked the pull requests to ensure that another person hasn't already submitted a fix.
• Added tests to ensure against regression.
• Updated the changelog

6️⃣ Thanks for contributing! 🙌

[stale]

This bug report has been automatically closed because it has not had recent activity. If this is still an active bug, please comment to reopen. Thank you for your contributions.

[cosmastech]

I think this is still a bug and should be reopened.