Check coverage matrix

[kesselb]

To do: Check if we could reduce the coverage matrix.

Why? The CI runs with coverage are usually a bit slower, and it takes some time for Codecov to process the results.

The initial reason to look into this topic were some issues when updating the Codecov action, leading to a "flanky" coverage score. This was resolved by settings the working-directory for the action: #9401 (comment)

Example: https://app.codecov.io/gh/nextcloud/mail/commit/63f31a2009e3c0a4aef9e56b9a471ddcaf40ea26

Workflow: https://github.com/nextcloud/mail/blob/63f31a2009e3c0a4aef9e56b9a471ddcaf40ea26/.github/workflows/test.yml

Unit:

mail/.github/workflows/test.yml

Lines 13 to 18 in 63f31a2

	matrix:
	php-versions: ['8.1', '8.2', '8.3']
	nextcloud-versions: ['master', 'stable30']
	include:
	- php-versions: '8.4'
	nextcloud-versions: 'master'

The coverage is calculated if php = 8.3
=> 2x coverage

Integration:

mail/.github/workflows/test.yml

Lines 64 to 81 in 63f31a2

	matrix:
	php-versions: ['8.3']
	nextcloud-versions: ['master']
	db: ['sqlite', 'mysql', 'pgsql']
	cache: ['nocache']
	include:
	- php-versions: 8.1
	nextcloud-versions: master
	db: 'sqlite'
	cache: 'redis'
	- php-versions: 8.2
	nextcloud-versions: master
	db: 'mysql'
	cache: 'redis'
	- php-versions: 8.2
	nextcloud-versions: stable30
	db: 'mysql'
	cache: 'redis'

The coverage is calculated if db = mysql
=> 4x coverage