Add deprecations warnings for RSpec 4

[benoittgt]

This issue is meant to follow the deprecation we will introduce in rspec-expectations 3.99.

• Ruby < 2.3 is no longer supported. (Phil Pirozhkov, Updates from rspec-dev (2020-11-22) #1230
  • Will be handled by bundler
• Remove should and should_not syntax (including one-liners). (Phil Pirozhkov, Scrape out monkey patching #1245)
  • Add deprecation warnings for features to be removed/changed in RSpec 4 #1301
• Turn strict_predicate_matchers on by default. (Phil Pirozhkov, Turn strict_predicate_matchers on by default #1277)
  • Do we want to print a warning when a predicate expectation fail?
• Remove deprecated LegacyMacherAdapter. (Phil Pirozhkov, Remove support for legacy RSpec matchers (pre 3) #1253)
  • Already deprecated
• Remove support for legacy RSpec matchers (pre 3). (Phil Pirozhkov, Remove support for legacy RSpec matchers (pre 3) #1253)
  • Legacy code
• Remove include_chain_clauses_in_custom_matcher_descriptions option and make it the default. (Phil Pirozhkov, Remove include_chain_clauses_in_custom_matcher_descriptions option #1279)
  • Needs to be deprecated when used
• Remove support for present-tense dynamic predicate. (Phil Pirozhkov, Remove support for present tense predicates #1286)
  • Needs to be deprecated when used
• Prevent implicit blocks (e.g blocks as values) from being used with block matchers. (Phil Pirozhkov, Restrict block matchers use with value expectations #1285)
  • Already deprecated
• Remove deprecated match_regex matcher. (Phil Pirozhkov, Remove a couple of remaining deprecations #1288)
  • Already deprecated
• Remove deprecated StartAndEndWith matcher base class. (Phil Pirozhkov, Remove a couple of remaining deprecations #1288)
  • Already deprecated