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

• Documentation PR: Disable transactional-update.timer on SLEM at bootstrap #8981

• 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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Re-run a test

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• 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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