Add the deprecation date to each deprecated entity [blocks: #3768, #4248, #4368, #4371]

[tautschnig]

This makes the date of deprecation visible in warning messages and also
makes tracking much easier: git grep DEPRECATED or any code review will
yield immediate insight on how long an entity has been deprecated
already.

The date is taken from the commit that first marked the entity as
deprecated.

• Each commit message has a non-empty body, explaining why the change was made.
• n/a Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• n/a The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• n/a Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• n/a My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[kroening]

clang-format isn't helping...

[tautschnig]

clang-format isn't helping...

Ack, but I figured we should rather remove many of them as soon as possible rather than bothering much with clang-format off or shortening the message.

[tautschnig]

FWIW: once this PR and (an agreed variant of) #4424 are merged I'd like to do a cleanup of all code that has gone past the six-months mark.

[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: 3222d89).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/105555269

[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: e70dcf2).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/105558661