Disable transactional-update.timer on SLEM at bootstrap

[m-czernek]

What does this PR change?

When bootstrapping SLEM, disable the SLEM's reboot timer so that it does not interfere with SUMA's timed work, e.g. reposyncing or other scheduled work.

GUI diff

No difference.

Before:

After:

• [ ] DONE

Documentation

TODO: After this PR's approval, this needs to be reflected in docs.

• Documentation issue was created: Link for SUSE Manager contributors, Link for community contributors.

• [ ] DONE

Test coverage

• No tests: already covered

• [ ] DONE

Links

Issue(s): # https://github.com/SUSE/spacewalk/issues/24251

• [ ] DONE

Changelogs

Make sure the changelogs entries you are adding are compliant with https://github.com/uyuni-project/uyuni/wiki/Contributing#changelogs and https://github.com/uyuni-project/uyuni/wiki/Contributing#uyuni-projectuyuni-repository

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• [ ] No changelog needed

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Re-run a test

If you need to re-run a test, please mark the related checkbox, it will be unchecked automatically once it has re-run:

• [ ] Re-run test "changelog_test"
• [ ] Re-run test "backend_unittests_pgsql"
• [ ] Re-run test "java_pgsql_tests"
• [ ] Re-run test "schema_migration_test_pgsql"
• [ ] Re-run test "susemanager_unittests"
• [ ] Re-run test "javascript_lint"
• [ ] Re-run test "spacecmd_unittests"

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