Use consistent icons

e.g.
https://matrix.org/ecosystem/clients/cinny/
https://matrix.org/ecosystem/servers/

~~issue is blocked by deciding which icons we want to use.~~

ACs:

- [ ] any icons we end up using should use REUSE annotations.
- [ ] Use filled icons consistently across the ecosystem
- [ ] Rename "Repository" to "Source Code" across the ecosystem
- [ ] Cleanup the now unused icons if they are not used elsewhere in the repository

Start by comparing the ecosystem pages linked in the issue and identify every icon and occurrence of “Repository” that needs to change. Confirm the selected filled icon set, add REUSE annotations to the icons in use, rename the label to “Source Code,” and remove only icons no longer used elsewhere.

Written by the indexing model from the issue text.