Update boto3 requirement from >=1.43.11 to >=1.43.15 in /src/slack-bot

Updates the requirements on [boto3](https://github.com/boto/boto3) to permit the latest version.
- [Release notes](https://github.com/boto/boto3/releases)
- [Commits](https://github.com/boto/boto3/compare/1.43.11...1.43.15)

---
updated-dependencies:
- dependency-name: boto3
  dependency-version: 1.43.15
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

src/slack-bot/requirements.txt

@@ -1,2 +1,2 @@
 slack_bolt>=1.28.0,<2.0
-boto3>=1.43.11
+boto3>=1.43.15