Azure-native error when deploying SRE with DSH Pulumi

[dsj976]

:white_check_mark: Checklist

• [x] I have searched open and closed issues for duplicates.
• [x] This is a problem observed when deploying a Data Safe Haven.
• [ ] I can reproduce this with the latest version.
• [x] I have read through the documentation.
• [x] This isn't an open-ended question (open a discussion if it is).

:computer: System information

• Operating System:
• Data Safe Haven version: branch tresa-testing

:package: Packages

List of packages

```none Paste list of packages here ```

:no_entry_sign: Describe the problem

I get the following error when running dsh sre deploy sandbox_sre:

Diagnostics:                                                                                                                        
  azure-native:operationalinsights:Workspace (sre_monitoring_log_analytics):                                                        
    error: azure-native:operationalinsights:Workspace resource 'sre_monitoring_log_analytics' has a problem: 'workspaceName' does   
not match expression '^[A-Za-z0-9][A-Za-z0-9-]+[A-Za-z0-9]$' 

:deciduous_tree: Log messages

Relevant log messages

```none Your log details here ```

:recycle: To reproduce

[jemrobinson]

This is caused by the use of underscores in the SRE name which need to be converted to - before they can be used as Azure resource names.