Update Api.getRawFile to make ref param optional[RUN-AS-IF-FOSS]

What does this MR do?

Following the api update #323450 (closed) the ref param is now optional and default to HEAD or default branch. Thus we can update the getRawFile method to match as described in #327200 (closed). This is also part of the effort to update the initial branch name in our frontend code https://gitlab.com/gitlab-org/gitlab/-/issues/324100.

Scans

grep -v -r --include=\*.js --include=\*.vue '^\s*\/\/\|^\s*\*' ./app | grep 'getRawFile\W'

./app/assets/javascripts/static_site_editor/services/load_source_content.js:  Api.getRawFile(projectId, sourcePath).then(({ data }) => ({
./app/assets/javascripts/pipeline_editor/graphql/resolvers.js:        rawData: Api.getRawFile(projectPath, path, { ref }).then(({ data }) => {
./app/assets/javascripts/api.js:  getRawFile(id, path, params) {

grep -v -r --include=\*.js --include=\*.vue '^\s*\/\/\|^\s*\*' ./ee/app | grep 'getRawFile\W'

./ee/app/assets/javascripts/groups/settings/compliance_frameworks/utils.js:    const { status } = await Api.getRawFile(`${group}/${project}`, file);

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

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Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
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Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
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Related to #327200 (closed)