Require ViennaRNA 2.7.0 and simplify logging synchronization

[Alexander-Mitrofanov]

Addresses the review request on #267. This follow-up targets audit/265-vienna-accessibility-thread-safety, so it contains only the requested revision to that PR.

ViennaRNA 2.7.0 is the first release providing vrna_log_lock_set(), the newest threading interface used here (release notes, pinned declaration). Require that version in configure and the conda environment, and remove the older-version logger guards. Configure also checks header versions and links the logger/prepared-window functions, including with --disable-pkg-config.

README, API comments, contributor guidance, the thread-safety audit, and ChangeLog now describe the supported range. The non-OpenMP serialization branch remains: OpenMP is an independent RNAlib build option, and raising the version does not make that fallback obsolete.

Validation on Linux with GCC 14.4.0:

• ViennaRNA 2.7.2 release and debug: make tests -j2 passed all seven suites in each build, including 75,901 assertions in 69 API cases.
• Six configure probes passed: valid dependency without pkg-config accepted; old metadata, old headers with/without pkg-config, missing logger declaration, and missing logger symbol rejected. Negative tests use isolated fixtures, including a header reporting version 2.6.4.
• Actual ViennaRNA 2.7.0 release build: make tests -j4 passed the same seven suites and 75,901 assertions. The standalone setup-locked 4 3 diagnostic matched 1,011,504 probabilities exactly with zero parameter or callback-metadata mismatches. The audit records the minimum-library build details.
• Installation, an external consumer exercising accessibility, and all 69 standalone public-header checks passed.
• Independent source review and git diff --check passed.