Document that the ccm server-version properties must be fully qualified

[nikagra]

The ccm server-version properties are documented in several places, none of which says the value
must be fully qualified. A partial version is accepted but makes every ccm create re-query S3 for
the newest patch, which slows the integration tests down substantially (see DRIVER-920, and #1043 /
#1044 which close the hole in make).

Places to state the rule, plus a one-line why:

• scylla-4.x: README-dev.md:25 ("Export SCYLLA_VERSION or CASSANDRA_VERSION to pin specific
  server versions before invoking") — this is the doc that invites SCYLLA_VERSION=2026.2;
  CONTRIBUTING.md:285-286; osgi-tests/README.md:55-56.
• scylla-3.x: CONTRIBUTING.md:105-115, which shows -Dscylla.version= and
  -Dcassandra.version= without a format note.

Two stale claims in the same 4.x paragraph, worth correcting while editing it:

• CONTRIBUTING.md:286 says ccm.version "defaults to 3.11.0". It is 4.0.0
  (CcmBridge.java:72).
• The same paragraph and CONTRIBUTING.md:430 reference -Dccm.scylla=true, removed in
  721a6db59b; the flag is now -Dccm.distribution=scylla.

Found while auditing version resolution for DRIVER-924 / DRIVER-925.