Update upper case strings

Looking for the pattern `>[A-Z]{2}` in the file https://github.com/vector-im/element-android/blob/develop/vector/src/main/res/values/strings.xml, and got 22 results.

This is a problem since those Strings can be used at several places and also on other projects.

TODO:

- [ ] Consider all those strings, and update them to first letter up only, or full lower case, depending on the case. No change can be acceptable too (for instance "OK" will stay "OK")
- [ ] Do not update translations, they will be updated through Weblate as usual
- [ ] Update the string usage to capitalize them if applicable. In XML, `android:textAllCaps="true"` can be added for instance. Buttons already have this attribute in the style
- [ ] Update the docs:
- [ ] https://github.com/vector-im/element-android/blob/main/docs/design.md#text: new added strings must not be imported if capitalized
- [ ] https://github.com/vector-im/element-android/blob/main/docs/pull_request.md: capitalized strings have to be checked during the PR review