Set readOnlyRootFilesystem as true in Controller Deployment

[khrm]

Setting Controller's Deployment security context readOnlyRootFilesystem to true to increase the security and to avoid being flagged by the security scanner.

Changes

Submitter Checklist

As the author of this PR, please check off the items in this checklist:

• [ ] Has Docs included if any changes are user facing
• [ ] Has Tests included if any functionality added or changed
• [x] Follows the commit message standard
• [x] Meets the Tekton contributor standards (including functionality, content, code)
• [ ] Release notes block below has been updated with any user facing changes (API changes, bug fixes, changes requiring upgrade notices or deprecation warnings)
• [ ] Release notes contains the string "action required" if the change requires additional action from users switching to the new release

Release Notes

Set readOnlyRootFilesystem as true in Watcher's Controller Deployment.

[tekton-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: wlynch

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files: - ~~[OWNERS](https://github.com/tektoncd/chains/blob/main/OWNERS)~~ [wlynch] Approvers can indicate their approval by writing `/approve` in a comment Approvers can cancel approval by writing `/approve cancel` in a comment

[wlynch]

/lgtm