Update qiskit-serverless requirement in /scripts/nb-tester

Updates the requirements on [qiskit-serverless](https://github.com/Qiskit/qiskit-serverless) to permit the latest version.
- [Release notes](https://github.com/Qiskit/qiskit-serverless/releases)
- [Commits](Qiskit/qiskit-serverless@v0.25.4...v0.26.1)

---
updated-dependencies:
- dependency-name: qiskit-serverless
  dependency-version: 0.26.1
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

scripts/nb-tester/requirements.txt

@@ -5,7 +5,7 @@ qiskit[all]~=2.1.2
 qiskit-ibm-runtime~=0.40.1
 qiskit-ibm-transpiler[ai-local-mode]~=0.14.2
 qiskit-aer~=0.17
-qiskit-serverless~=0.26.0
+qiskit-serverless~=0.26.1
 qiskit-ibm-catalog~=0.10.0
 qiskit-addon-sqd~=0.12.0
 qiskit-addon-utils~=0.1.1