How to remove notice at login
This is a preference you can set, but it's not exposed in the web interface.  There's a section of civicrm.settings.php that documents it.  In case you've upgraded from an older version of CiviCRM that doesn't have it, I'm including it here:

 // Opt-out of announcements by the CiviCRM core team for releases, paid services, events, etc. Separate each preferred optout category with a comma:                                                              
 //   "offers": paid service offers                 
 //   "asks": requests for donations or membership signup/renewal to CiviCRM                             
 //   "releases": major release announcements       
 //   "events": announcements of local/national upcoming events                                          
 $civicrm_setting['CiviCRM Preferences']['communityMessagesUrl'] = 'https://alert.civicrm.org/alert?prot=1&amp;ver={ver}&amp;uf={uf}&amp;sid={sid}&amp;lang={lang}&amp;co={co}&amp;optout=offers,asks'; 


More documentation is available at in the CiviCRM Administration Guide, see page Customization > Override Settings.