[FEATURE] Use `repo: 'varnish60lts'` by default

[myii]

Is your feature request related to a problem?

Describe the solution you'd like

Just updated to use the latest pre-salted images in 8c5107c. The test pillar now uses:

varnish-formula/test/salt/pillar/varnish.sls

Line 10 in 8c5107c

repo: 'varnish60lts' # to specify another custom repo than default (varnish40)

Consider updating map.jinja to use these by default, changing these lines:

varnish-formula/varnish/ng/map.jinja

Line 10 in 8c5107c

'repo': 'varnish40',

varnish-formula/varnish/ng/map.jinja

Line 26 in 8c5107c

'repo': 'varnish40',

This probably entails using a BREAKING CHANGE commit.

Describe alternatives you've considered

Additional context