RoomCreationIntroCell now conforms to Themable protocol

[mgcm]

RoomCreationIntroCell now conforms to Themable protocol, making this cell type able to be updated when the system changes from light to dark theme and vice-versa. The update method was already implemented but just not called because the runtime did not match the Themable protocol on the TableViewCell object.

This fixes issue #7554 , as shown in video below:

Signed-off-by: Milton Moura miltonmoura@gmail.com

Pull Request Checklist

• [X] I read the contributing guide
• [X] UI change has been tested on both light and dark themes, in portrait and landscape orientations and on iPhone and iPad simulators
• [X] Accessibility has been taken into account.
• [X] Pull request is based on the develop branch
• [X] Pull request contains a changelog file in ./changelog.d
• [X] You've made a self review of your PR
• [X] Pull request includes screenshots or videos of UI changes
• [X] Pull request includes a sign off

[codecov[bot]]

Codecov Report

Patch and project coverage have no change.

Comparison is base (b2945a4) 12.21% compared to head (fedaf6b) 12.21%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #7555 +/- ## ======================================== Coverage 12.21% 12.21% ======================================== Files 1648 1648 Lines 163581 163581 Branches 67154 67154 ======================================== + Hits 19983 19985 +2 + Misses 142953 142951 -2 Partials 645 645 ``` | Flag | Coverage Δ | | |---|---|---| | uitests | `55.05% <ø> (-0.01%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=vector-im#carryforward-flags-in-the-pull-request-comment) to find out more. | [Impacted Files](https://app.codecov.io/gh/vector-im/element-ios/pull/7555?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=vector-im) | Coverage Δ | | |---|---|---| | [...ells/RoomCreationIntro/RoomCreationIntroCell.swift](https://app.codecov.io/gh/vector-im/element-ios/pull/7555?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=vector-im#diff-UmlvdC9Nb2R1bGVzL1Jvb20vVGltZWxpbmVDZWxscy9Sb29tQ3JlYXRpb25JbnRyby9Sb29tQ3JlYXRpb25JbnRyb0NlbGwuc3dpZnQ=) | `0.00% <ø> (ø)` | | ... and [2 files with indirect coverage changes](https://app.codecov.io/gh/vector-im/element-ios/pull/7555/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=vector-im)

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