Respect the m.change_password capability

[hughns]

From https://spec.matrix.org/v1.16/client-server-api/#mchange_password-capability. This isn't listed as a requirement for OAuth 2.0 aware MSC3824, but does make the UX more sensible.

Pull Request Checklist

• I read the contributing guide
• UI change has been tested on both light and dark themes, in portrait and landscape orientations and on iPhone and iPad simulators
• Accessibility has been taken into account.

• Pull request is based on the develop branch

• Pull request contains a changelog file in ./changelog.d
• You've made a self review of your PR
• Pull request includes screenshots or videos of UI changes
• Pull request includes a sign off

[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 16.96%. Comparing base (ede2db6) to head (1657e06).
⚠️ Report is 1 commits behind head on develop.

Files with missing lines	Patch %	Lines
Riot/Modules/Settings/SettingsViewController.m	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #7990      +/-   ##
===========================================
- Coverage    16.96%   16.96%   -0.01%     
===========================================
  Files         1766     1766              
  Lines       122112   122112              
  Branches     49184    49183       -1     
===========================================
- Hits         20719    20715       -4     
- Misses      100717   100721       +4     
  Partials       676      676              

Flag	Coverage Δ
unittests	8.81% <0.00%> (-0.01%)	⬇️

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[pixlwave]

Looks reasonable to me, thanks. Just needs a changelog 👍