2.2.1 - upgrade to TZDB 2023b; immutable TimeZone; support SAMD21 again

• 2.2.1 (2023-03-24, TZDB version 2023b)
  • Actually regenerate the zonedb* files to 2023b.
• 2.2.0 (2023-03-24, TZDB version 2023b)
  • Upgrade TZDB from 2022g to 2023b
    • 2023a skipped because it came out only a day earlier.
    • 2023a: https://mm.icann.org/pipermail/tz-announce/2023-March/000077.html
      • Egypt now uses DST again, from April through October.
      • This year Morocco springs forward April 23, not April 30.
      • Palestine delays the start of DST this year.
      • Much of Greenland still uses DST from 2024 on.
      • America/Yellowknife now links to America/Edmonton.
      • tzselect can now use current time to help infer timezone.
      • The code now defaults to C99 or later.
      • Fix use of C23 attributes.
    • 2023b: https://mm.icann.org/pipermail/tz-announce/2023-March/000078.html
      • Lebanon delays the start of DST this year.
  • Bug Fix
    • Change arguments for TimeZone::forMinutes() from uint8_t to
      uint16_t.
  • Breaking Change
    • Make TimeZone effectively immutable, by removing setStdOffset()
      and setDstOffset() methods.
    • Client applications should create a new TimeZone object using
      TimeZone::forTimeOffset(std, dst) and overwrite the old one.
    • See Migrating to v2.2.
  • Add support for Seeed Studio XIAO M0 (SAMD21).
    • Required updating zoneino/compat.h to clobber the broken definition
      of FPSTR() in Seeeduino 1.8.3.
  • Simplify the handling of Rule.LETTER.
    • Encode all letters as an index into ZoneContext.letters array, not
      just LETTER which are only a single character.
    • On 8-bit AVR:
      • Increases BasicZoneProcessor by ~200 bytes when 1-2 zones are
        used. But flash remains the same when the full TZ database is
        used.
      • No change to ExtendedZoneProcessor for 1-2 zones. But decreases
        flash usage by ~300 bytes when the full TZ database is used.
    • The small increase in flash is worth it because this greatly
      simplifies the complicated code surrounding LETTER that was difficult
      to understand and maintain.
  • Unify Basic and Extended zoneinfo encoding.
    • Change encoding of basic::ZoneRule.deltaCode,
      basic::ZoneEra.deltaCode, and basic::ZoneEra.offsetCode to be
      identical their counterparts in extended::ZoneXxx.
    • Merge BasicBrokers.h and ExtendedBrokers.h into just Brokers.h.
  • Restructure zonedb directories
    • Lift ace_time/internal/testing/tzonedb[x] database files
      to ace_time/tzonedb[x].
    • Lift ace_time/zonedb[x] database files to ace_time/zonedb[x].
    • These changes are transparent to client apps because the C++
      namespaces of these files are unchanged.
    • Lift ace_time/internal/Zone*.h files into new src/zoneinfo/
      directory. These are the classes that describe the *zonedb*
      databases.
  • Add ZonedDateTime::offsetDateTime()
    • Returns the underlying OffsetDateTime inside the ZonedDateTime.
    • Analogous to ZonedDateTime::localDateTime().
  • Always generate anchor rules in zonedb.
    • Allows ExtendedZoneProcessor to work over all years [0,10000)
      even with truncated zonedb (e.g. [2000,2100)).
    • Accuracy is guaranteed only for the requested interval (e.g.
      [2000,2100).
    • But the code won't crash outside of that interval.