Align paymentsMessage and paymentsSidebar with flex instead of float

[luixxiul]

Fixes #10153

Auditors: @cezaraugusto

Test Plan:

1. Open about:preferences#payments
2. Enable and disable Payments
3. Make sure that the margin between the title wrapper and the content does not change

Submitter Checklist:

• Submitted a ticket for my issue if one did not already exist.
• Used Github auto-closing keywords in the commit message.
• Added/updated tests for this change (for new code or code which already has tests).
• Ran git rebase -i to squash commits (if needed).
• Tagged reviewers and labelled the pull request as needed.

Test Plan:

Reviewer Checklist:

Tests

• Adequate test coverage exists to prevent regressions
• Tests should be independent and work correctly when run individually or as a suite ref
• New files have MPL2 license header

[codecov-io]

Codecov Report

Merging #10154 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master   #10154   +/-   ##
=======================================
  Coverage   52.81%   52.81%           
=======================================
  Files         227      227           
  Lines       20195    20195           
  Branches     3234     3234           
=======================================
  Hits        10665    10665           
  Misses       9530     9530

Flag	Coverage Δ
#unittest	52.81% <100%> (ø)	⬆️

Impacted Files	Coverage Δ
.../components/preferences/payment/disabledContent.js	100% <100%> (ø)	⬆️

[cezaraugusto]

much better! such flexbox very amazing ++