chore: github/README update release month (#4359)

# Description

July is definitely over.

## Checklist

Please put an x inside each checkbox to indicate that you've read and followed it: `[ ]` -> `[x]`

If this is a documentation change, only the first checkbox must be filled (you can delete the others if you want).

- [x] I/we have read the [GoToSocial contribution guidelines](https://codeberg.org/superseriousbusiness/gotosocial/src/branch/main/CONTRIBUTING.md).
- [ ] I/we have discussed the proposed changes already, either in an issue on the repository, or in the Matrix chat.
- [x] I/we have not leveraged AI to create the proposed changes.
- [ ] I/we have performed a self-review of added code.
- [ ] I/we have written code that is legible and maintainable by others.
- [ ] I/we have commented the added code, particularly in hard-to-understand areas.
- [ ] I/we have made any necessary changes to documentation.
- [ ] I/we have added tests that cover new code.
- [ ] I/we have run tests and they pass locally with the changes.
- [ ] I/we have run `go fmt ./...` and `golangci-lint run`.

Reviewed-on: https://codeberg.org/superseriousbusiness/gotosocial/pulls/4359
Co-authored-by: daenney <git@noreply.sourcery.dny.nu>
Co-committed-by: daenney <git@noreply.sourcery.dny.nu>

.github/README.md

@@ -2,4 +2,4 @@
 
 This is a mirror. You can find us on https://codeberg.org/superseriousbusiness/gotosocial.
 
-We will **stop mirroring** to Github after 0.20 is released. This will likely be sometime **in July**.
+We will **stop mirroring** to Github after 0.20 is released. This will likely be sometime **in August**.