Solved:
# composer why-not flarum/core v1.0.1
tpokorra/flarum-ext-post-notification 0.4.2 requires flarum/core (>0.1.0-beta.16 <=1.0.0)
# composer remove tpokorra/flarum-ext-post-notification
./composer.json has been updated
Running composer update tpokorra/flarum-ext-post-notification
Loading composer repositories with package information
Updating dependencies
Lock file operations: 0 installs, 0 updates, 1 removal
- Removing tpokorra/flarum-ext-post-notification (0.4.2)
Writing lock file
Installing dependencies from lock file (including require-dev)
Package operations: 0 installs, 0 updates, 1 removal
- Removing tpokorra/flarum-ext-post-notification (0.4.2)
Generating autoload files
135 packages you are using are looking for funding.
Use the `composer fund` command to find out more!
# composer update --prefer-dist --no-dev -a
Loading composer repositories with package information
Updating dependencies
Lock file operations: 0 installs, 1 update, 0 removals
- Upgrading flarum/core (v1.0.0 => v1.0.1)
Writing lock file
Installing dependencies from lock file
Package operations: 0 installs, 1 update, 0 removals
- Downloading flarum/core (v1.0.1)
- Upgrading flarum/core (v1.0.0 => v1.0.1): Extracting archive
Generating optimized autoload files
135 packages you are using are looking for funding.
Use the `composer fund` command to find out more!