Remove unused Azure DevOps boards sync workflow (#11868)

# Description

Removes the `devops-boards.yaml` GitHub workflow, which synchronized
issues to Azure DevOps work items. The workflow is no longer used and is
already disabled in the GitHub repository.

Changes:

- Deleted `.github/workflows/devops-boards.yaml`.
- Removed the `.github/workflows/devops-boards.yaml` entry from the
non-code paths list in `.github/workflows/__changes.yml`.
- Removed the `danhellem` entry from `.cspellignore` (only referenced by
the deleted workflow).

Note: Repo-level cleanup outside this PR is still needed: the `issues`
environment variables (`AZURE_SP_DEVOPS_SYNC_CLIENT_ID`,
`AZURE_SP_DEVOPS_SYNC_TENANT_ID`), the Azure SP federated credential,
and the SP's Azure DevOps permissions can be removed.

## Type of change

- This pull request is a minor refactor, code cleanup, test improvement,
or other maintenance task and doesn't change the functionality of Radius
(issue link optional).

## Contributor checklist
Please verify that the PR meets the following requirements, where
applicable:

- An overview of proposed schema changes is included in a linked GitHub
issue.
    - [ ] Yes
    - [x] Not applicable
- A design document is added or updated under `eng/design-notes/` in
this repository, if new APIs are being introduced.
    - [ ] Yes
    - [x] Not applicable
- The design document has been reviewed and approved by Radius
maintainers/approvers.
    - [ ] Yes
    - [x] Not applicable
- A PR for
[resource-types-contrib](https://github.com/radius-project/resource-types-contrib/)
is created, if resource types or recipes are affected by the changes in
this PR.
    - [ ] Yes
    - [x] Not applicable
- A PR for [dashboard](https://github.com/radius-project/dashboard/) is
created, if the Radius Dashboard is affected by the changes in this PR.
    - [ ] Yes
    - [x] Not applicable
- A PR for the [documentation
repository](https://github.com/radius-project/docs) is created, if the
changes in this PR affect the documentation or any user facing updates
are made.
    - [ ] Yes
    - [x] Not applicable

---------

Signed-off-by: Brooke Hamilton <45323234+brooke-hamilton@users.noreply.github.com>
Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>

Author: brooke-hamilton

.github/workflows/__changes.yml

@@ -33,7 +33,6 @@ on:
           .github/workflows/close-stale-prs.yml
           .github/workflows/devcontainer-feature-release.yaml
           .github/workflows/devcontainer-feature-test.yaml
-          .github/workflows/devops-boards.yaml
           .github/workflows/publish-*.yaml
           .github/workflows/purge-*.yaml
           .github/workflows/radius-bot.yaml