Added placeholder to VPN plan expiration message

[spylogsster]

Resolves brave/brave-browser#22914

Submitter Checklist:

• I confirm that no security/privacy review is needed, or that I have requested one
• There is a ticket for my issue
• Used Github auto-closing keywords in the PR description above
• Wrote a good PR/commit description
• Squashed any review feedback or "fixup" commits before merge, so that history is a record of what happened in the repo, not your PR
• Added appropriate labels (QA/Yes or QA/No; release-notes/include or release-notes/exclude; OS/...) to the associated issue
• Checked the PR locally: npm run test -- brave_browser_tests, npm run test -- brave_unit_tests, npm run lint, npm run gn_check, npm run tslint
• Ran git rebase master (if needed)

Reviewer Checklist:

• A security review is not needed, or a link to one is included in the PR description
• New files have MPL-2.0 license header
• Adequate test coverage exists to prevent regressions
• Major classes, functions and non-trivial code blocks are well-commented
• Changes in component dependencies are properly reflected in gn
• Code follows the style guide
• Test plan is specified in PR before merging

After-merge Checklist:

• The associated issue milestone is set to the smallest version that the
  changes has landed on
• All relevant documentation has been updated, for instance:
  • https://github.com/brave/brave-browser/wiki/Deviations-from-Chromium-(features-we-disable-or-remove)
  • https://github.com/brave/brave-browser/wiki/Proxy-redirected-URLs
  • https://github.com/brave/brave-browser/wiki/Fingerprinting-Protections
  • https://github.com/brave/brave-browser/wiki/Brave%E2%80%99s-Use-of-Referral-Codes
  • https://github.com/brave/brave-browser/wiki/Custom-Headers
  • https://github.com/brave/brave-browser/wiki/Web-Compatibility-Exceptions-in-Brave
  • https://github.com/brave/brave-browser/wiki/QA-Guide
  • https://github.com/brave/brave-browser/wiki/P3A

Test Plan:

Live test

1. Fresh profile
2. Enable VPN / SKU / etc - set environment as staging
3. Login to account.bravesoftware.com and sign in
4. Check panel (click VPN button next to hamburger) to have VPN in the message
5. Should have Looks like your most recent payment for Brave Firewall + VPN did not go through. (Brave Firewall + VPN is the part we care about)

Storybook test

1. Launch latest storybook link
2. Go to VPN > Panels > Error Subscription Expired
3. Verify the string matches our expectations (ex: $1 is replaced)

[brave-builds]

A Storybook has been deployed to preview UI for the latest push

[brave-builds]

A Storybook has been deployed to preview UI for the latest push

[bsclifton]

I didn't test this live... but the code changes look good! I updated test plan and also storybook so that we can see the replacement happening in Storybook (in the component). That works great!

[bsclifton]

OK found a way to test the expired text live and verified it looks great! Will update test plan

[brave-builds]

A Storybook has been deployed to preview UI for the latest push