Draft: Add troubleshooting step to update db_key_size

What does this MR do?

Adds a troubleshooting documentation about a problem that can result from !43950 (merged) (discussion) combined with a too short db_key_base.

This aggregates the support team's snippets "Bad Secrets check 2" & "Bad Secret Cleaner".

Related issues

• ZD 148744 (internal) (long-term solution & snippets)
• ZD 183620 (internal)

Author's checklist (required)

• Follow the Documentation Guidelines and Style Guide.
• If you have Developer permissions or higher:
  • Ensure that the product tier badge is added to doc's h1.
  • Apply the documentation label, plus:
    • The corresponding DevOps stage and group labels, if applicable.
    • development guidelines when changing docs under doc/development/*, CONTRIBUTING.md, or README.md.
    • development guidelines and Documentation guidelines when changing docs under development/documentation/*.
    • development guidelines and Description templates (.gitlab/*) when creating/updating issue and MR description templates.
  • Assign the designated Technical Writer.

Do not add the feature, frontend, backend, ~"bug", or database labels if you are only updating documentation. These labels will cause the MR to be added to code verification QA issues.

When applicable:

• Update the permissions table.
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• Add GitLab's version history note(s).
• Add/update the feature flag section.

Review checklist

All reviewers can help ensure accuracy, clarity, completeness, and adherence to the Documentation Guidelines and Style Guide.

1. Primary Reviewer

• Review by a code reviewer or other selected colleague to confirm accuracy, clarity, and completeness. This can be skipped for minor fixes without substantive content changes.

2. Technical Writer

• Technical writer review. If not requested for this MR, must be scheduled post-merge. To request for this MR, assign the writer listed for the applicable DevOps stage.
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  • If working on UI text, add the corresponding UI Text scoped label.
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For suggestions that you are confident don't need to be reviewed, change them locally and push a commit directly to save others from unneeded reviews. For example:

• Clear typos, like this is a typpo.
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3. Maintainer

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app/assets/javascripts/blob/template_selectors/ci_syntax_yaml_selector.js

+import FileTemplateSelector from '../file_template_selector';
+import initDeprecatedJQueryDropdown from '~/deprecated_jquery_dropdown';
+
+export default class BlobCiSyntaxYamlSelector extends FileTemplateSelector {
+  constructor({ mediator }) {
+    super(mediator);
+    this.config = {
+      key: 'gitlab-ci-yaml',
+      name: '.gitlab-ci.yml',
+      pattern: /(.gitlab-ci.yml)/,
+      type: 'gitlab_ci_syntax_ymls',
+      dropdown: '.js-gitlab-ci-syntax-yml-selector',
+      wrapper: '.js-gitlab-ci-syntax-yml-selector-wrap',
+    };
+  }
+
+  initDropdown() {
+    initDeprecatedJQueryDropdown(this.$dropdown, {
+      data: this.$dropdown.data('data'),
+      filterable: true,
+      selectable: true,
+      search: {
+        fields: ['name'],
+      },
+      clicked: options => this.reportSelectionName(options),
+      text: item => item.name,
+    });
+  }
+}