chore: bump default theme version to 1.5.0 (#4289)

#### What type of PR is this?

/area theme
/milestone 2.8.x

#### What this PR does / why we need it:

升级默认主题版本至 1.5.0

https://github.com/halo-dev/theme-earth/releases/tag/v1.5.0

#### Does this PR introduce a user-facing change?

```release-note
None
```