[atlas][backport 3.2] Increases number of pages

[pblottiere]

Description

This PR increases a hardcoded value limiting the maximum number of pages in the atlas.

Backport of #7399 for 3.2.

Checklist

• Commit messages are descriptive and explain the rationale for changes
• Commits which fix bugs include fixes #11111 in the commit message next to the description
• Commits which add new features are tagged with [FEATURE] in the commit message
• Commits which change the UI or existing user workflows are tagged with [needs-docs] in the commit message and contain sufficient information in the commit message to be documented
• I have read the QGIS Coding Standards and this PR complies with them
• This PR passes all existing unit tests (test results will be reported by travis-ci after opening this PR)
• New unit tests have been added for core changes
• I have run the scripts/prepare-commit.sh script before each commit

[pblottiere]

@nyalldawson qgis_meshlayerrenderertest is failing over and over, but it's clearly not related to this PR. Do you agree to merge?

[nyalldawson]

Yes - that mesh layer renderer test has been flaky recently

[pblottiere]

Yes

Thanks :)