chore: bump dependabot PR limit for cargo from 5 to 15 (#19730)

## Which issue does this PR close?

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- N/A

## Rationale for this change

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Occasionally we do PRs to bump a bunch of versions in `Cargo.lock`,
recent example:

- #19667

Ideally this should be handled by dependabot already. I suspect the
default limit of 5 is preventing dependabot from creating all the PRs it
needs; this causes it to error and those version bumps are "lost".

- I say "lost" but dependabot can pick it up on the next run... if it
doesn't error again

See an example from a dependabot run:

<img width="703" height="118" alt="image"
src="https://github.com/user-attachments/assets/f8ae6b70-6fe1-4613-8c60-28564c23188b"
/>

From the logs:

```text
 +-------------------------------------------------------+
|          Changes to Dependabot Pull Requests          |
+---------+---------------------------------------------+
| created | insta ( from 1.45.0 to 1.46.0 )             |
| created | tracing ( from 0.1.43 to 0.1.44 )           |
| created | syn ( from 2.0.111 to 2.0.113 )             |
| created | async-compression ( from 0.4.35 to 0.4.36 ) |
| created | object_store ( from 0.12.4 to 0.13.0 )      |
| created | serde_json ( from 1.0.145 to 1.0.148 )      |
| created | bigdecimal ( from 0.4.9 to 0.4.10 )         |
| created | clap ( from 4.5.53 to 4.5.54 )              |
| created | libc ( from 0.2.177 to 0.2.179 )            |
| created | tokio ( from 1.48.0 to 1.49.0 )             |
| created | tokio-util ( from 0.7.17 to 0.7.18 )        |
| created | sqllogictest ( from 0.28.4 to 0.29.0 )      |
+---------+---------------------------------------------+
```

We expect to have all these PRs created (this was for the run 5 days
ago), but only these were created on that day:

- #19645
- #19644
- #19643

And considering these PRs were still open at the time:

- #19544
- #19325

We can see it hit the 5 limit.

> Dependabot default behavior:
>
> - If five pull requests with version updates are open, no further pull
requests are raised until some of those open requests are merged or
closed.
> - Security updates have a separate, internal limit of ten open pull
requests which cannot be changed.

-
https://docs.github.com/en/code-security/dependabot/working-with-dependabot/dependabot-options-reference#open-pull-requests-limit-

## What changes are included in this PR?

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Bump the limit to 15. We might have an appetite for increasing it more,
15 was chosen arbitrarily.

## Are these changes tested?

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Dependabot only.

## Are there any user-facing changes?

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No.

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Author: Jefffrey

.github/dependabot.yml

@@ -23,6 +23,7 @@ updates:
       interval: weekly
     target-branch: main
     labels: [auto-dependencies]
+    open-pull-requests-limit: 15
     ignore:
       # major version bumps of arrow* and parquet are handled manually
       - dependency-name: "arrow*"